Calpactins: two distinct Ca++-regulated phospholipid- and actin-binding proteins isolated from lung and placenta

Three forms of calpactin, the 36,000 Mr Ca++-binding cytoskeletal protein, were isolated in large amounts from bovine lung and human placenta using cycles of calcium-dependent precipitation followed by solubilization with EGTA-containing buffers. Calpactin-I as a tetramer of heavy (36 kD) and light (11 kD) chains was the predominant form of calpactin isolated, however milligram amounts of the calpactin-I heavy chain monomer and calpactin-II, a related but distinct molecule, were also isolated by this method. Calpactin-II was characterized in some detail and found to bind two Ca++ ions with Kd's of 10 microM in the presence of phosphatidylserine. Both calpactin-I and -II were found to aggregate liposomes at micromolar Ca++ concentrations, suggesting that at least two phospholipid-binding sites are present on these molecules. Both calpactin monomers bind to and bundle actin filament at high (1 mM) but not low (less than 1 microM) Ca++ concentrations. Amino-terminal sequence analysis of a lower molecular mass variant of calpactin-II revealed that this protein was the previously identified human "lipocortin" molecule. Antibodies were elicited to calpactin-I and -II and the cell and subcellular distribution of each was compared. Calpactin-II was only present at high levels in tissues (lung, placenta) which contained high levels of calpactin-I. Other tissues (intestine) contained high calpactin-I and undetectable levels of calpactin-II. Double-label immunofluorescence microscopy on human fibroblasts revealed that, like calpactin-I, calpactin-II is present in a submembraneous reticular network, although the distribution of the two calpactins is not identical.     

Kinetic properties of ATP-dependent phosphofructokinase from grapefruit juice sacs: effect of TCA cycle intermediates.

Grapefruit juice sac ATP-PFK was studied kinetically for its substrates ATP and Fru-6-P at pH = 7.5. The Km for ATP is equal to 39.8 +/- 4.6 microM. ATP becomes inhibitory at concentrations above 80 microM. The Km for ATP is not affected by the addition of citrate (10 mM). For Fru-6-P, the saturation curve is sigmoidal, with an S0.5 equal to 0.17 +/- 0.03 mM, in the presence of Mg++ (2.5 mM) and ATP (1 mM). ATP-PFK shows a negative cooperativity at lower concentrations of Fru-6-P (h = 0.5), while higher concentrations of the substrate induce a positive cooperation (h = 1.5). The presence of citrate affects the S0.5 affinity value, but not the Vmax. The presence of citrate (10 mM) removes the cooperative effect at higher concentrations of the substrate, as h = 1.0. A theoretical Ki for citrate was calculated and equals 1.30 mM.     

Purification and characterization of bovine heart phosphoinositide-specific phospholipase C: kinetic analysis of the Ca2+ requirement and La3+ inhibition

Bovine heart contains multiple phosphoinositide-specific phospholipase C (PIC) activities separable by ion-exchange chromatography. One PIC activity was purified to apparent homogeneity and migrated as a single band of Mr 85,000 on SDS-PAGE. The purified PIC was characterized with sonicated suspensions of either pure phosphatidylinositol 4,5-bisphosphate (PIP2) or phosphatidylinositol (PI) as substrates. At pH 7, apparent Vmax and Km values were higher for PIP2 than for PI, but the value of Vmax/Km was similar for the two substrates. PIC required Ca2+ for the hydrolysis of either PI or PIP2, and increasing free Ca2+ concentrations from 20 to 300 nM saturated PIC activity. The requirement of Ca2+ for PIC activity and the sensitivity of PIC to Ca2+ concentrations in the physiological range suggested the ion may be a cofactor. The PIC reaction mechanism was determined by two-substrate kinetic analysis; the data fit a model in which PIC contained single sites for Ca2+ and phosphoinositide, and utilized a rapid-equilibrium, random-order ternary mechanism for phosphoinositide hydrolysis. The KCa value for either PI or PIP2 hydrolysis was approximately 30 nM, suggesting resting intracellular free Ca2+ concentrations are sufficient to saturate the Ca2+ site of PIC. La3+ was used as a calcium analogue to modulate PIC activity. Low concentrations of LaCl3 (0.01-0.3 microM) inhibited PIC activity competitively with respect to calcium, consistent with a Ca2+ binding site on the enzyme.     

Differential stimulation of protein kinase C activity by phorbol ester or calcium/phosphatidylserine in vitro and in intact synaptosomes.

Activation of protein kinase C (PKC) by phorbol 12-myristate 13-acetate (PMA) was compared with calcium/phosphatidylserine (Ca/PS). The substrate specificity of PKC was more limited with PS/PMA. Substrates could be divided into three overlapping groups according to their relative level of phosphorylation: C1, relatively preferred substrates with Ca/PS, included dephosphin, histone, and peptide GS1-10. C2, relatively preferred with PS/PMA, included myelin basic protein and MARCKS. C3, substrates independent of activators. PS/PMA altered the Vmax of PKC for substrate, and decreased the Km for Mg2+. Differential substrate phosphorylation by PS/PMA also occurred for PKC isozymes resolved by hydroxylapatite chromatography and was most dramatic for PKC-alpha, which could no longer phosphorylate histone or GS1-12. Differential activities of PKC were also observed in synaptosol and in intact synaptosomes where PMA stimulated phosphorylation of MARCKS, but not dephosphin. It was further shown that dephosphin was indeed a substrate of PKC in the intact synaptosomes by use of a repolarization-dependent dephosphin phosphorylation assay. The differential PKC activities could also be distinguished by inhibitors. H-7 was equipotent, palmitoylcarnitine did not inhibit in vitro C2 phosphorylation, but inhibited dephosphin in intact synaptosomes, and sphingosine did not inhibit C1 substrates and was without effect on dephosphin in intact synaptosomes. Therefore PS/PMA alters or limits the substrate specificity of PKC, leading to a differential substrate phosphorylation in vitro and in intact synaptosomes and differential inhibitor sensitivity. The pattern of protein phosphorylation observed after PKC activation in intact cells will therefore be dependent upon the activator.     

Phosphorylation of glycerol and dihydroxyacetone in Acetobacter xylinum and its possible regulatory role.

Extracts of Acetobacter xylinum catalyze the phosphorylation of glycerol and dihydroxyacetone (DHA) by adenosine 5'-triphosphate (ATP) to form, respectively, L-alpha-glycerophosphate and DHA phosphate. The ability to promote phosphorylation of glycerol and DHA was higher in glycerol-grown cells than in glucose- or succinate-grown cells. The activity of glycerol kinase in extracts is compatible with the overall rate of glycerol oxidation in vivo. The glycerol-DHA kinase has been purified 210-fold from extracts, and its molecular weight was determined to be 50,000 by gel filtration. The glycerol kinase to DHA kinase activity ratio remained essentially constant at 1.6 at all stages of purification. The optimal pH for both reactions was 8.4 to 9.2. Reaction rates with the purified enzyme were hyperbolic functions of glycerol, DHA, and ATP. The Km for glycerol is 0.5 mM and that for DHA is 5 mM; both are independent of the ATP concentration. The Km for ATP in both kinase reactions is 0.5 mM and is independent of glycerol and DHA concentrations. Glycerol and DHA are competitive substrates with Ki values equal to their respective Km values as substrates. D-Glyceraldehyde and l-Glyceraldehyde were not phosphorylated and did not inhibit the enzyme. Among the nucleotide triphosphates tested, only ATP was active as the phosphoryl group donor. Fructose diphosphate (FDP) inhibited both kinase activities competitively with respect to ATP (Ki= 0.02 mM) and noncompetitively with respect to glycerol and DHA. Adenosine 5'-diphosphate (ADP) and adenosine 5'-monophosphate (AMP) inhibited both enzymic activities competitively with respect to ATP (Ki (ADP) = 0.4 mM; Ki (AMP) =0.25 mM). A. xylinum cells with a high FDP content did not grow on glycerol. Depletion of cellular FDP by starvation enabled rapid growth on glycerol. It is concluded that a single enzyme from A. xylinum is responsible for the phosphorylation of both glycerol and DHA. This as well as the sensitivity of the enzyme to inhibition by FDP and AMP suggest that it has a regulatory role in glycerol metabolism.     

Lactose and D-galactose metabolism in Staphylococcus aureus. III. Purification and properties of D-tagatose-6-phosphate kinase

D-Tagatose-6-phosphate kinase, an inducible enzyme that functions in the metabolism of lactose and D-galactose in Staphylococcus aureus, was purified about 300-fold from an extract of D-galactose-grown cells. The enzyme catalyzed the nucleoside triphosphate-dependent phosphorylation of both D-tagatose 6-phosphate and D-fructose 6-phosphate. Although the Vmax values were equal for these two substrates, the apparent Km values differed by 10,000-fold, being 16 micro M for D-tagatose 6-phosphate and 150 mM for D-fructose 6-phosphate. The purified enzyme was free from the constitutive D-fructose-6-phosphate kinase. Phosphoryl donors used by D-tagatose-6-phosphate kinse, listed in order of decreasing rates at saturating concentrations were GTP, UTP ITP ATP, CTP, and TTP; the Km values were 0.38, 0.91, 0.17, 0.16, 18, and 20 mM, respectively. The enzyme appeared to be nonallosteric; it exhibited Michaelis-Menten kinetics and was not inhibited by high concentrations of MgATP. However, it was activated 3- to 4-fold by 33.3 mM K+, NH4+, Rb+, and Cs+, and was inhibited 31 to 65% by 33.3 mM Na+ and Li+. It was inactivated reversibly by the thiol reagent, N-ethylmaleimide. The subunit molecular weight was estimated to be 52,000, and the native enzyme appeared to be a dimer with a sedimentation coefficient of 6.8 S. Data on stability, pH optimum, and inducibility of the enzyme are also presented.     

N-demethylation of N-nitrosodimethylamine catalyzed by purified rat hepatic microsomal cytochrome P-450: isozyme specificity and role of cytochrome b5

Metabolism of the potent hepatocarcinogen N-nitrosodimethylamine (NDMA) was evaluated in reconstituted monooxygenase systems containing each of 11 purified rat hepatic cytochrome P-450 isozymes. The reaction has an absolute requirement for cytochrome P-450, NADPH-cytochrome P-450 reductase, and NADPH, as well as a partial dependence on dilauroylphosphatidylcholine. Of the cytochrome P-450 isozymes evaluated, only cytochrome P-450j, purified from livers of ethanol- or isoniazid-treated rats, had high catalytic activity for the N-demethylation of NDMA. At substrate concentrations of 0.5 and 5 mM, rates of NDMA metabolism to formaldehyde catalyzed by cytochrome P-450j were at least 15-fold greater than the rates obtained with any of the other purified isozymes. At the pH optimum (approximately 6.7) for the reaction, the Km,app and Vmax were 3.5 mM and 23.9 nmol/min/nmol cytochrome P-450j, respectively. With hepatic microsomes from ethanol-treated rats, which contain induced levels of cytochrome P-450j, the Km,app and Vmax were 0.35 mM and 3.9 nmol/min/nmol cytochrome P-450, respectively. Inclusion of purified cytochrome b5 in the reconstituted system containing cytochrome P-450j caused a six-fold decrease in Km,app (0.56 mM) of NDMA demethylation with little or no change in Vmax (29.9 nmol/min/nmol cytochrome P-450j). Trypsin-solubilized cytochrome b5, bovine serum albumin, or hemoglobin had no effect on the kinetic parameters of the reconstituted system, indicating a specific effect of intact cytochrome b5 on the Km,app of the reaction. These results demonstrate high isozyme specificity in the metabolism of NDMA to an ultimate carcinogen and further suggest an important role for cytochrome b5 in this biotransformation process.     

The interaction of aromatic amino acids with rat liver phenylalanine hydroxylase

We have examined the interaction of hepatic phenylalanine hydroxylase with the phenylalanine analogs, tryptophan and the diastereomers of 3-phenylserine (beta-hydroxyphenylalanine). Both isomers of phenylserine are substrates for native phenylalanine hydroxylase at pH 6.8 and 25 degrees C, when activity is measured with the use of the dihydropteridine reductase assay coupled with NADH in the presence of the synthetic cofactor, 6-methyl-5,6,7,8-tetrahydropterin. However, while erythro-phenylserine exhibits simple Michaelis-Menten kinetics (Km = 1.2 mM, Vmax = 1.2 mumol/min X min) under these conditions, the threo isomer exhibits strong positive cooperativity (S0.5 = 4.8 mM Vmax = 1.4 mumol/min X mg, nH = 3). Tryptophan also exhibits cooperativity under these conditions (S0.5 = 5 mM, Vmax = 1 mumol/min X mg, nH = 3). The presence of 1 mM lysolecithin results in a hyperbolic response of phenylalanine hydroxylase to tryptophan (Km = 4 mM, Vmax = 1 mumol/min X mg) and threo-phenylserine (Km = 2 mM, Vmax = 1.4 mumol/min X mg). erythro-Phenylserine is a substrate for native phenylalanine hydroxylase in the presence of the natural cofactor, L-erythro-tetrahydrobiopterin (BH4) (Km = 2 mM, Vmax 0.05 mumol/min X mg, nH = 2). Preincubation of phenylalanine hydroxylase with erythro-phenylserine results in a 26-fold increase in activity upon subsequent assay with BH4 and erythro-phenylserine, and hyperbolic kinetic plots are observed. In contrast, both threo-phenylserine and tryptophan exhibit negligible activity in the presence of BH4 unless the enzyme has been activated. The product of the reaction of phenylalanine hydroxylase with either isomer of phenylserine was identified as the corresponding p-hydroxyphenylserine by reaction with sodium periodate and nitrosonaphthol. With erythro-phenylserine, the hydroxylation reaction is tightly coupled (i.e. 1 mol of hydroxyphenylserine is formed for every mole of tetrahydropterin cofactor consumed), while with threo-phenylserine and tryptophan the reaction is largely uncoupled (i.e. more cofactor consumed than product formed). Erythro-phenylserine is a good activator, when preincubated with phenylalanine hydroxylase (A0.5 = 0.2 mM), with a potency about one-third that of phenylalanine (A0.5 = 0.06 mM), while threo-phenylserine (A0.5 = 6 mM) and tryptophan (A0.5 approximately 10 mM) are very poor activators. Addition of 4 mM tryptophan or threo-phenylserine or 0.2 mM erythro-phenylserine to assay mixtures containing BH4 and phenylalanine results in a dramatic increase in the hydroxylation at low concentrations of phenylalanine.(ABSTRACT TRUNCATED AT 400 WORDS)     

Polylysine and polyamine stimulation of the phosphatidylinositol kinases of amphibian oocyte membranes

Phosphatidylinositol kinase present in Xenopus laevis oocyte membranes catalyzes the formation of phosphatidylinositol 4-phosphate using phosphatidylinositol and ATP as substrates while the activity of a second enzyme, phosphatidylinositol-4-phosphate kinase, results in the synthesis of phosphatidylinositol 4,5-bisphosphate. Large (Mr greater than 20,000) homopolymers of L-lysine or L-ornithine can stimulate the activity of both of these enzymes by at least 2-fold at 10-20 microM concentrations. Under similar conditions poly-L-arginine fails to stimulate the reaction causing a partial inhibition. Smaller polylysine (25 lysines) or lysine-rich oligopeptides such as one corresponding to the last 14 amino acids of the carboxyl end of c-Ki-ras 2 protein produce appreciable stimulation of phosphatidylinositol but at concentrations of 300-500 microM. Spermine and spermidine at millimolar concentrations also stimulate exogenous phosphatidylinositol phosphorylation. The amino-glycoside antibiotic neomycin has a biphasic effect, stimulating the phosphatidylinositol kinase at concentrations below 0.5 mM and strongly inhibiting at higher concentrations. Polylysine also moderately stimulates the loss of radioactivity of phosphatidylinositol-4-[32P] phosphate observed in oocyte membranes. Polylysine and polyornithine do not change the apparent Km for ATP of the phosphatidylinositol kinase but increase the Vmax of the reaction.     

Protein kinase C-dependent phosphorylation of profilin is specifically stimulated by phosphatidylinositol bisphosphate (PIP2)

Calf spleen profilin is shown to be an in vitro substrate of purified human placental protein kinase C (PKC), with an apparent Km of 4 microM. Phosphatidylinositol bisphosphate (PIP2) was an effective activator of the profilin phosphorylation by PKC and caused a maximum 13-fold increase of Vmax with a half maximal effect at 40 micrograms/ml. The action of PIP2 was not mimicked by phosphatidylserine, phosphatidic acid or phosphatidylinositol, whereas phosphatidylinositol monophosphate was slightly stimulatory. By contrast, protein kinase C-dependent phosphorylation of histone type III-S, myelin basic protein or lipocortin-I was not affected by PIP. It is suggested that PIP2 modifies the nature of the profilin-PKC interactions.     

Kinetic characterization of outer-ring deiodinase activity (ORD) in the liver, gill and retina of the killifish Fundulus heteroclitus

Conversion of T4 to T3 is the first step in TH action and deiodinases are the major determinants of TH tissue availability and disposal. We here report the kinetic characterization of the outer-ring deiodinating (ORD) enzymes in the liver, gill and retina of sea water-adapted killifish, by using both rT3 and T4 as substrates. In liver, by using rT3, we detected a high Km (84 nM) and a low Km (1.3 nM) component with kinetic characteristics similar to mammalian deiodinases DI and DII. In contrast, T4-ORD only generated a low Km (0.5 nM) component. As judged by its Vmax (920 fmol 125I/mg per h) this DII enzyme is very abundant, approximately five and 20 times higher than that found in trout liver and hypothyroid rat, respectively. Kinetic analysis in killifish gill showed only one enzymatic component, with a high rT3 Km (430 nM) and a relatively low Vmax (4.3 pmol 125I/mg per h). Our results in killifish retina show the expression of a T4-low Km (0.6 nM) deiodinase with high cofactor requirements akin to the mammalian DII. The Vmax value for this enzyme is 182 fmol 125I/mg per h, five times lower than the one found in killifish liver, but comparable to that in hypothyroid rat pituitary. The biochemical similarities between fish and mammalian deiodinases could reflect their high conservation during vertebrate evolution and thus their importance in the regulation of thyroid hormone action.     

Kinetics of phosphorylation of Na+/K(+)-ATPase by protein kinase C

The kinetics of phosphorylation of an integral membrane enzyme, Na+/K(+)-ATPase, by calcium- and phospholipid-dependent protein kinase C (PKC) were characterized in vitro. The phosphorylation by PKC occurred on the catalytic alpha-subunit of Na+/K(+)-ATPase in preparations of purified enzyme from dog kidney and duck salt-gland and in preparations of duck salt-gland microsomes. The phosphorylation required calcium (Ka approximately 1.0 microM) and was stimulated by tumor-promoting phorbol ester (12-O-tetradecanoylphorbol 13-acetate) in the presence of a low concentration of calcium (0.1 microM). PKC phosphorylation of Na+/K(+)-ATPase was rapid and plateaued within 30 min. The apparent Km of PKC for Na+/K(+)-ATPase as a substrate was 0.5 microM for dog kidney enzyme and 0.3 microM for duck salt-gland enzyme. Apparent substrate inhibition of PKC activity was observed at concentrations of purified salt-gland Na+/K(+)-ATPase greater than 1.0 microM. Phosphorylation of purified kidney and salt-gland Na+/K+ ATPases occurred at both serine and threonine residues. The 32P-phosphopeptide pattern on 15% sodium dodecyl sulfate-polyacrylamide gel electrophoresis after hydroxylamine cleavage of pure 32P-phosphorylated alpha subunit was the same for the two sources of enzyme, which suggests that the phosphorylation sites are similar. The results indicate that Na+/K(+)-ATPase may serve as a substrate for PKC phosphorylation in intact cells and that the Na+/K(+)-ATPase could be a useful in vitro model substrate for PKC interaction with integral membrane proteins.     

Purification of an inositol (1,3,4,5)-tetrakisphosphate 3-phosphatase activity from rat liver and the evaluation of its substrate specificity.

Hepatic inositol (1,3,4,5)-tetrakisphosphate 3-phosphatase activity was detected in a 100,000 x g soluble fraction and a detergent-solubilized particulate fraction. Activity in both fractions increased up to 40-fold after anion-exchange chromatography due to removal of endogenous inhibitors (Hodgson, M.E., and Shears, S.B. (1990) Biochem. J. 267, 831-834); at this stage the detergent-solubilized particulate activity comprised over 90% of total activity. The particulate phosphatase was further purified by affinity chromatography using heparin-agarose and red-agarose. The latter column resolved two peaks of enzyme activity (designated 1 and 2 by their order of elution from the column). Their proportions varied between experiments, but peak 2 generally predominated and so this was further purified by hydroxylapatite chromatography. The final preparation was typically 38,000-fold purified with a 7% yield. The apparent molecular mass of this enzyme was 66 kDa, as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and gel filtration. The enzyme had little or no affinity for the following: inositol (1,3,4,6)-tetrakisphosphate, inositol (1,3,4)-trisphosphate, inositol (1,3)-bisphosphate, inositol (3,4)-bisphosphate, and para-nitrophenylphosphate. At pH 7.4 the Km for inositol (1,3,4,5)-tetrakisphosphate was 130 nM and the Vmax was 4250 nmol/mg protein/min. The purified enzyme also dephosphorylated inositol (1,3,4,5,6)-pentakisphosphate to inositol (1,4,5,6)-tetrakisphosphate (Km = 40 nM, Vmax = 211 nmol/mg protein/min), and inositol hexakisphosphate to at least five isomers of inositol pentakisphosphate (Km = 0.3 nM, Vmax = 12 nmol/mg protein/min). The latter affinity is the highest yet defined for an enzyme involved in inositol phosphate metabolism. Determinations of IC50 values, and Dixon plots, revealed that with the (1,3,4,5)-tetrakisphosphate as substrate, the pentakis- and hexakisphosphates were potent competitive inhibitors; the Ki values (25 and 0.5 nM, respectively) were similar to their substrate Km values. The kinetic properties of this enzyme, as well as estimates of the cellular levels of its potential substrates, indicate that inositol pentakisphosphate and inositol hexakisphosphate are likely to be the preferred substrates in vivo.     

Actin kinase: a protein kinase that phosphorylates actin of fragmin-actin complex.

Actin of fragmin-actin complex is phosphorylated by an endogenous kinase from plasmodium of Physarum polycephalum. The phosphorylation abolishes the nucleation and capping activities of fragmin-actin complex. The kinase has been purified and termed actin kinase [Furuhashi, K. & Hatano, S. (1990) J. Cell Biol. 111, 1081-1087]. Enzymatic properties of the purified actin kinase were studied in detail. Actin kinase exhibited the highest activity under conditions physiological for the plasmodium (30 mM KCl, 6 mM MgCl2, pH 7.0). The Vmax and the Km of the enzyme for ATP were about 83 mumol/min/mg and 25 microM, respectively. The Km for fragmin-actin complex was 190 nM. The purified actin kinase phosphorylated actin of fragmin-actin complex at a constant rate regardless of Ca2+ concentration. Similarly, 2 microM cAMP, 2 microM cGMP, 2 micrograms/ml calmodulin in the presence of Ca2+ or 1 mM GTP showed no effect on the activity of the purified enzyme. Actin kinase did not phosphorylate histone H1, H2B, alpha-casein, or beta-casein, suggesting that actin kinase is a new kind of protein kinase which specifically phosphorylates actin of the fragmin-actin complex.     

Na+-like effect of imidazole on the phosphorylation of (Na+ + K+)-ATPase

A high basal level of phosphorylation (approx. 70% of the optimal Na+-dependent phosphorylation level) is observed in 50 mM imidazole-HCl (pH 7.0), in the absence of added Na+ and K+ and the presence of 10-100 microM Mg2+. In 50 mM Tris-HCl (pH 7.0) the basal level is only 5%, irrespective of the Mg2+ concentration. Nevertheless, imidazole is a less effective activator of phosphorylation than Na+ (Km imidazole-H+ 5.9 mM, Km Na+ 2 mM under comparable conditions). Imidazole-activated phosphorylation is strongly pH dependent, being optimal at pH less than or equal to 7 and minimal at pH greater than or equal to 8, while Na+-activated phosphorylation is optimal at pH 7.4. This suggests that imidazole-H+ is the activating species. Imidazole facilitates Na+-stimulated phosphorylation. The Km for Na+ decreases from 0.63 mM at 5 mM imidazole-HCl to 0.21 mM at 50 mM imidazole-HCl (pH 7; 0.1 mM Mg2+ in all cases). Imidazole-activated phosphorylation is more sensitive to inhibition by K+ (I50 = 12.5 microM) than Na+-activated phosphorylation (I50 = 180 microM). Mg2+ antagonizes activation by imidazole-H+ and also inhibition by K+. The Ki value for Mg2+ (approx. 0.3 mM) is the same for the two antagonistic effects. Tris buffer (pH 7.0) inhibits imidazole-activated phosphorylation with an I50 value of 30 mM in 50 mM imidazole-HCl (pH 7.0) plus 0.1 mM Mg2+. We conclude that imidazole-H+, but not Tris-H+, can replace Na+ as an activator of ATP-dependent phosphorylation, primarily by shifting the E2----E1 transition to the right, leading to a phosphorylating E1 conformation which is different from that in Tris buffer.     

The mechanism of activation of heart fructose 6-phosphate,2-kinase:fructose-2,6-bisphosphatase

Partially purified fructose-6-P,2-kinase:fructose-2,6-bisphosphatase from beef heart was phosphorylated by cAMP protein kinase. The phosphorylated fructose-6-P,2-kinase shows lower Km for Fru-6-P (43 versus 105 microM) and for ATP (0.55 versus 1.3 mM) but no change in the Vmax, compared to those for unphosphorylated enzyme. There was no detectable change in Km or Vmax of fructose-2,6-bisphosphatase activity by the phosphorylation. These changes in heart fructose-6-P,2-kinase were in direct contrast to previous results for the liver isozyme in which phosphorylation led to inhibition of the kinase activity and activation of the phosphatase activity.     

The pH dependence of the cardiac sarcolemmal Ca2(+)-transporting ATPase: evidence that the Ca2+ translocator bears a doubly negative charge

The pH dependence of the Ca2(+)-transporting ATPase of bovine cardiac sarcolemma was determined in a membrane vesicle preparation. The maximal velocity (Vmax) at saturating external Ca2+ showed a sigmoidal pH dependence with maximal values in the 6.0-6.5 range, a half-maximal value at 7.2 and minimal (less than or equal to 15%) values at pH greater than or equal to 8.0. The apparent affinity for Ca2+ (1/Km) varied over 10(4)-fold for 6.0 less than or equal to pH less than or equal to 8.5, increasing with increasing pH. Plots of log(1/Km) vs. pH were biphasic. In the acid range (6.0 less than or equal to pH less than or equal to 7.2), a slope of 2.6 was observed for the calmodulin-activated form of the pump. For 7.2 less than or equal to pH less than or equal to 8.5, a slope of 0.5 was observed. At pH 7.4, the Km is approx. 48 +/- 19 nM. The Ca2+ pump of cardiac sarcoplasmic reticulum in the same preparation had a Km of 304 +/- 115 nM and showed a similar pH dependence except that the slope in the acid range was 1.7. When calmodulin was removed from the sarcolemmal pump, its Km was raised to approx. 1.0 microM, the slope in the acid range was reduced to 1.7 and the Vmax was markedly reduced. The results are explicable in terms of a model in which each of the two Ca2+ binding sites on the pump contains two buried COO- groups responsible for high affinity. The Km effect is explained by 2 H+ vs. 1 Ca2+ competition for occupation of each of the two cytoplasmically-oriented translocators (4 H+ vs. 2 Ca2+). The Vmax effect is explained by counter-transport of H+. The findings are considered in terms of the published amino acid sequence of the cardiac sarcolemmal pump and recent site-directed mutagenesis vs. function studies identifying the Ca2+ binding site in the skeletal sarcoplasmic reticulum pump. The kinetic data are also applied to pump behavior under conditions of ischemia and acidosis.     

Calcium transport and phosphorylated intermediate of (Ca2+ + Mg2+)-ATPase in plasma membranes of rat liver

We have identified and characterized calcium transport and the phosphorylated intermediate of the (Ca2+ + Mg2+)-ATPase in plasma membrane vesicles prepared from rat liver. The calcium transport did not absolutely require the presence of oxalate and was completely inhibited by 1 microM of ionophore A23187. Oxalate, which serves as a trapping agent in calcium uptake of skeletal muscle and liver microsomes, was not absolutely required to maintain the net accumulation of calcium. The Vmax and Km for calcium uptake were 35.2 +/- 10.1 pmol of calcium/mg of protein/min, and 17.6 +/- 2.5 nM of free calcium, respectively. Ten mM magnesium was required for the maximal accumulation of calcium. Substitution of 5 and 10 mM ADP, CTP, GTP, and UTP for ATP could not support calcium uptake. The calcium uptake was not affected by 0.5 mM ouabain, 20 mM azide, or 2 micrograms/ml of oligomycin but was inhibited in a dose-dependent fashion by vanadate, with a Ki of approximately 20 microM for vanadate. The substrate affinities and specificities of this calcium-transport activity suggest that it is closely associated with the (Ca2+ + Mg2+)-ATPase reported in the plasma membranes of liver (Lotersztajn, S., Hanoune, J., and Pecker, F. (1981) J. Biol. Chem. 256, 11209-11215). A calcium-stimulated and magnesium-dependent phosphoprotein was also demonstrated in the same membrane vesicles. The free calcium concentration at which its phosphorylation was half-maximal was 15.5 +/- 5.6 nM. Sodium fluoride, ouabain, sodium azide, oligomycin, adriamycin, and N,N'-dicyclohexylcarbodiimide did not affect its formation while vanadate at 100 microM inhibited the calcium-dependent phosphorylation by approximately 60%. The properties of this phosphoprotein suggest that it may be the phosphorylated intermediate of the (Ca2+ + Mg2+)-ATPase in the plasma membranes of rat liver.     

Intestinal brush border calcium uptake in spontaneously hypertensive rats and their genetically matched WKY rats

The current studies were designed to characterize calcium transport by intestinal brush border membrane in the spontaneously hypertensive rat (SHR) and normotensive control, the Wistar-Kyoto (WKY) rat. The biochemical and functional purity of the intestinal brush border membranes in SHR and WKY rats was validated by marker enzymes and the ability to transiently transport D-glucose in the presence of Na+ gradient. Calcium transport into duodenal and jejunal vesicles represented a minor binding component and transmembrane movement as evident by initial rate studies, A23187 studies, and lanthanum displacement experiments. Initial rate and time course of calcium uptake was lower in SHR compared with WKY rats. Kinetic analysis of calcium uptake by the jejunum (total uptake minus binding component) showed a Vmax of 6.98 +/- 0.2 and 1.8 +/- 0.2 nmol/mg protein/7 sec in WKY rats and SHR, respectively (P less than 0.001), whereas Km values were 0.76 +/- 0.04 and 0.87 +/- 0.1 mM for WKY rats and SHR, respectively. Similar kinetic analysis of calcium uptake by the duodenal segments showed a Vmax of 10.3 +/- 0.8 and 2.8 +/- 0.2 nmol/mg protein/7 sec in WKY rats and SHR, respectively (P less than 0.01). Km values were 0.7 +/- 0.2 and 0.3 +/- 0.06 mM (P greater than 0.05). Vmax of calcium uptake in the 2-week-old rats (prehypertensive period) was 6.0 +/- 0.3 and 3.53 +/- 0.3 nmol/mg protein/7 sec in WKY rats and SHR, respectively (P less than 0.001), whereas Km values were 0.60 +/- 0.07 and 0.5 +/- 0.01 mM, respectively. These results suggest that calcium binding and uptake by duodenal and jejunal intestinal brush border membranes of SHR is significantly decreased compared with WKY rats. The decrease in transmembrane calcium uptake is secondary to decrease in Vmax and is present before the appearance of hypertension, implying a genetically determined defect in calcium uptake in intestinal brush border membranes of the SHR.     

A sensitive method for the detection of beta-galactosidase in transfected mammalian cells.

A sensitive method has been developed for the detection of E. coli beta-galactosidase in transfected HeLa cells. The chromogenic substrate, CPRG (chlorophenol red-beta-D-galactopyranoside), was compared with ONPG (o-nitrophenyl-beta-D-galactopyranoside) by kinetic analysis with purified beta-galactosidase. The Km for CPRG was 1.35 mM and the Vmax was 21.4, whereas the Km for ONPG was 2.42 and the Vmax was 41.1. CPRG at 8.0 mM (6-fold Km) gave 86% of the Vmax and was used as the standard concentration for quantitation of enzyme levels. The Vmax for CPRG was half that for ONPG, and chlorophenol red has an extinction coefficient that is 21-fold higher than o-nitrophenol; these factors make CPRG about 10-fold greater in sensitivity for the quantitation of enzyme levels. The use of Nonidet P-40 to lyse the cells and the use of CPRG as substrate permitted the rapid detection of low levels of enzyme production from transfected human cells that could not be detected using ONPG.