Extracellular hydrolysis of formyl peptides and subsequent uptake of liberated amino acids by alveolar macrophages

The mechanism of accumulation of radioactive label from fNle-Leu-[3H]Phe by guinea pig alveolar macrophages was investigated. The binding of fNle-Leu-[3H]Phe to macrophages reached equilibrium within 5 min at 4 degrees C, but equilibrium could not be achieved at temperatures where fNle-Leu-Phe stimulated superoxide anion production is observed (e.g., 21-23 degrees C). At this temperature a rapid phase of initial binding of fNle-Leu-[3H]Phe to its receptor was followed by continued accumulation of cell-associated radioactivity which was linear and was dependent on the extracellular pH, i.e., the rate increased as the pH was lowered from pH 8 to pH 6. Examination for possible intracellular hydrolysis of fNle-Leu-[3H]Phe revealed the presence of extensive amounts of [3H]phenylalanine, both cell-associated and in the medium. The increases in cell-associated [3H]phenylalanine correlated in time and pH with cell-associated radioactivity that was accumulated after stimulation with fNle-Leu-[3H]Phe. The addition of 1 mM unlabelled phenylalanine blocked the long term accumulation of label from fNle-Leu-[3H]Phe by macrophages. 1 mM phenylalanine had no measureable effect on fNle-Leu-Phe stimulated O2- production, fNle-Leu-[3H]Phe hydrolysis or on fNle-Leu-[3H]Phe binding to its receptor. These results indicated that the long term accumulation of radioactivity by alveolar macrophages was due to extracellular hydrolysis of fNle-Leu-[3H]Phe followed by transport of liberated [3H]phenylalanine into the cells. A high affinity (Km = 3.56 X 10(-8) M) transport system for phenylalanine was measured in alveolar macrophages, which was not stimulated by the addition of fNle-Leu-Phe. The extracellular hydrolysis of fNle-Leu-[3H]Phe could not be attributed to release of macrophage enzymes into the medium. The responsible proteinase appears to be membrane bound and has a Km for the hydrolysis of fNle-Leu-[3H]Phe of 2.6 X 10(-7) M which is similar to the Kd (1.5 X 10(-7) M) for fNle-Leu-Phe binding. Taken together, these data suggest that for the alveolar macrophage: (1) formyl peptides are not internalized by a receptor-mediated process; (2) a surface proteinase can catalyze the hydrolysis of formyl peptides; and (3) [3H]phenylalanine formed by fNle-Leu-[3H]Phe hydrolysis is transported into the interior of the macrophage.     

Effects of pH on allosteric and catalytic properties of the guanosine cyclic 3',5'-phosphate stimulated cyclic nucleotide phosphodiesterase from calf liver

We have investigated effects of pH on the catalytic and allosteric properties of the cGMP-stimulated cyclic nucleotide phosphodiesterase purified from calf liver. In the "activated" state, i.e., with 0.5 microM [3H]cAMP plus 1 microM cGMP or at saturating substrate concentrations (250 microM [3H]cAMP or [3H]cGMP), hydrolysis was maximal at pH 7.5-8.0 in assays of different pH. Hydrolysis of concentrations of substrate not sufficient to saturate regulatory sites and below the apparent Michaelis constant (Kmapp), i.e., 0.5 microM [3H]cAMP or 0.01 microM [3H]cGMP, was maximal at pH 9.5. Although hydrolysis of 0.5 microM [3H]cAMP increased with pH from 7.5 to 9.5, cGMP stimulation of cAMP hydrolysis decreased. As pH increased or decreased from 7.5, Hill coefficients (napp) and Vmax for cAMP decreased. Thus, assay pH affects both catalytic (Vmax) and allosteric (napp) properties. Enzyme was therefore incubated for 5 min at 30 degrees C in the presence of MgCl2 at various pHs before assay at pH 7.5. Prior exposure to different pHs from pH 6.5 to 10.0 did not alter the Vmax or cGMP-stimulated activity (assayed at pH 7.5). Incubation at high (9.0-10.0) pH did, in assays at pH 7.5, markedly increase hydrolysis of 0.5 microM [3H]cAMP and reduce Kmapp and napp. After incubation at pH 10, hydrolysis of 0.5 microM [3H]cAMP was maximally increased and was similar in the presence or absence of cGMP. Thus, after incubation at high pH, the phosphodiesterase acquires characteristics of the cGMP-stimulated form. Activation at high pH occurs at 30 degrees C but not 5 degrees C, requires MgCl2, and is prevented but not reversed by ethylenediaminetetraacetic acid.(ABSTRACT TRUNCATED AT 250 WORDS)     

Hydrolysis of chylomicron arachidonate and linoleate ester bonds by lipoprotein lipase and hepatic lipase

Chylomicrons labeled with [3H]arachidonic and [14C]linoleic acid were incubated with bovine milk lipoprotein lipase or rat postheparin plasma, containing both lipoprotein lipase and hepatic lipase. During incubation with bovine lipoprotein lipase, [3H]arachidonic acid was released from chylomicron triacylglycerols at a slower rate than [14C]linoleic acid. Only small amounts of [14C]linoleic acid were found as 1,2(2,3)-diacylglycerols, whereas a transient accumulation as [14C]monoacylglycerols was observed. In contrast, significantly more [3H]arachidonic acid was found as 1,2(2,3)-diacylglycerols than as monoacylglycerols at all time intervals investigated. The initial pattern of triacylglycerol hydrolysis by postheparin plasma was similar to that of bovine lipoprotein lipase. However, in contrast to the results obtained with bovine lipoprotein lipase, little [3H]1,2(2,3)-diacylglycerol accumulated. The addition of antiserum to hepatic lipase increased the amount of 3H found in 1,2(2,3)-diacylglycerols and inhibited the formation of free [3H]arachidonic acid. The antiserum also caused a significant inhibition of the hydrolysis of [3H]-but not of [14C]triacylglycerol. With regard to chylomicron phospholipids, the rate of hydrolysis of [14C]linoleoyl phosphatidylcholine with milk lipoprotein lipase was twofold higher than that of the [3H]arachidonyl phosphatidylcholine. However, the hepatic lipase of postheparin plasma had similar activity towards the two phosphatidylcholine species. Postheparin plasma rapidly hydrolyzed chylomicron 3H-labeled and 14C-labeled phosphatidylethanolamine to the same degree, and lipoprotein lipase similarly hydrolyzed 3H-labeled and 14C-labeled phosphatidylethanolamine at approximately equal rates. Antiserum to hepatic lipase inhibited the postheparin plasma hydrolysis of phosphatidylethanolamine and 3H-labeled phosphatidylcholine by about 60%, but the 14C-labeled phosphatidylcholine by only 27%.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of pH on Muscarinic Acetylcholine Receptors from Rat Brainstem

The effect of hydrogen ion concentration on ligand binding to muscarinic acetylcholine receptors was studied in membranes isolated from rat brainstem. The binding of [3H]methylscopolamine was constant between pH 7 and 10. The affinity, but not the number, of [3H]methylscopolamine binding sites decreased below pH 7; at pH 4 little binding was detected. When brainstem membranes were incubated at various pH levels from 3 to 11 for 1 h and then returned to pH 8, [3H]methylscopolamine binding affinity was restored to control levels. Carbamylcholine binding affinity was also depressed in media of low pH. However, this decrease was permanent after a 1-h incubation at pH 4 (i.e. carbamylcholine affinity was not restored on raising the pH to 8). The capacity of a guanine nucleotide to affect carbamylcholine was also abolished by a 1-h incubation at pH 4, and was not restored by raising the pH. The guanine nucleotide-dependent regulatory protein may be irreversibly inactivated or dissociated from the receptor at low pH. The receptor's binding subunit, on the other hand, appears to be much less sensitive to hydrogen ion concentration.     

Exogenous myristic acid acylates proteins in cultured rat hepatocytes

Fatty acid acylation is a functionally important modification of proteins. In the liver, however, acylated proteins remain largely unknown. This work was aimed at investigating fatty acid acylation of proteins in cultured rat hepatocytes. Incubation of these cells with [9,10-3H] myristic acid followed by two-dimensional electrophoresis separation of the delipidated cellular proteins and autoradiography evidenced the reproducible and selective incorporation of radioactivity from the precursor into 18 well-resolved proteins in the 10--120 kDa range and the 4--7 pH range. Radiolabeling of these proteins resulted from covalent linkage to the precursor [9,10-3H] myristic acid or to its elongation product, palmitic acid. The majority of the covalent linkages between the proteins and the fatty acids were broken by base hydrolysis, which indicated that the linkage was of thioester or ester-type. Only one of the studied proteins was attached to myristic acid via an amide linkage which resisted the basic treatment but was broken by acid hydrolysis. After incubation with [9,10-3H] palmitic acid, only two proteins previously detected with myristic acid were radiolabeled. Finally, the identified acylated proteins may be grouped into two classes: proteins involved in signal transduction (the alpha subunit of a heterotrimeric G protein and several small G proteins) and cytoskeletal proteins (cytokeratins, actin).     

Direct determination of acetyl-enzyme intermediate in the acetylcholinesterase-catalyzed hydrolysis of acetylcholine and acetylthiocholine

Acetylcholinesterase from Electrophorus electricus was acetylated during the hydrolysis of [3H]acetylcholine and [3H]acetylthiocholine. The steady state levels of [3H]acetyl-enzyme were measured at different pH and different concentrations of substrate. The maximum acetylation fraction [S)----infinity) at pH 7.0 in 0.5 M salt was 0.65 with acetylcholine as substrate and 0.57 with acetylthiocholine as substrate. Acetylation is faster than deacetylation. The fraction of acetyl-enzyme was not affected by pH which indicates that acetylation and deacetylation are equally affected by changes in pH. This results supports the concept that acetylation and deacetylation involve similar mechanisms.     

Effects of fatty acids on activity of cGMP-stimulated cyclic nucleotide phosphodiesterase from calf liver

Effects of fatty acids, prostaglandins, and phospholipids on the activity of purified cGMP-stimulated cyclic nucleotide phosphodiesterase from calf liver were investigated. Prostaglandins A2, E1, E2, F1 alpha, and F2 alpha, thromboxane B2, and most phospholipids were without effect; lysophosphatidylcholine was a potent inhibitor. Several saturated fatty acids (carbon chain length 14-24), at concentrations up to 1 mM, had little or no effect on hydrolysis of 0.5 microM [3H]cGMP or 0.5 microM [3H]cAMP with or without 1 microM cGMP. In general, unsaturated fatty acids were inhibitory, except for myristoleic and palmitoleic acids which increased hydrolysis of 0.5 microM [3H]cAMP. The extent of inhibition by cis-isomers correlated with the number of double bonds. Increasing concentrations of palmitoleic acid from 10 to 100 microM increased hydrolysis of [3H]cAMP with maximal activation (60%) at 100 microM; higher concentrations were inhibitory. Palmitoleic acid inhibited cGMP hydrolysis and cGMP-stimulated cAMP hydrolysis with IC50 values of 110 and 75 microM, respectively. Inhibitory effects of palmitoleic acid were completely or partially prevented by equimolar alpha-tocopherol. Palmitelaidic acid, the trans isomer, had only slightly inhibitory effects. The effects of palmitoleic acid (100 microM) were dependent on substrate concentration. Activation was maximal with 1 microM [3H]cAMP and was reduced with increasing substrate; with greater than 10 microM cAMP, palmitoleic had no effect. Inhibition of cGMP hydrolysis was maximal at 2.5 microM cGMP and was reduced with increasing cGMP; at greater than 100 microM cGMP palmitoleic acid increased hydrolysis slightly. Palmitoleic acid did not affect apparent Km or Vmax for cAMP hydrolysis, but increased the apparent Km (from 17 to 60 microM) and Vmax for cGMP hydrolysis with little or no effect on the Hill coefficient for either substrate. These results suggest that certain hydrophobic domains play an important role in modifying the catalytic specificity of the cGMP-stimulated phosphodiesterase for cAMP and cGMP.     

Chronic Lithium Treatment Impairs Phosphatidylinositol Hydrolysis in Membranes from Rat Brain Regions

Membranes prepared from rat brain regions were used to measure the receptor-coupled and/or guanine nucleotide-binding protein (G protein)-mediated hydrolysis of exogenous [3H]phosphatidylinositol ([3H]PI). Guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S) and NaF (in the presence of AlCl3) caused concentration-dependent stimulations of [3H]PI hydrolysis, supporting the conclusion that G proteins mediating [3H]PI hydrolysis can be activated in this preparation. Neither of these responses was altered by in vitro incubation with 8 mM LiCl, but both were reduced in hippocampal, striatal, and cortical membranes from rats that had been treated with lithium for 4 weeks compared with controls. Two cholinergic agonists, carbachol and pilocarpine, induced no hydrolysis of [3H]PI unless GTP gamma S was also present, in which case each equally stimulated [3H]PI hydrolysis above that obtained with GTP gamma S alone. In the presence of GTP gamma S several excitatory amino acid agonists stimulated [3H]PI hydrolysis to an extent similar to that of carbachol. After chronic lithium treatment, [3H]PI hydrolysis stimulated by carbachol was significantly attenuated, but the response to quisqualate was unaffected. Therefore, lithium added in vitro does not have an effect on cholinergic receptor- or G protein-mediated [3H]PI hydrolysis, but each of these is reduced by chronic lithium treatment. Because exogenous [3H]PI was provided as the substrate, it is evident that the inhibitory effect of chronic lithium treatment cannot be due to substrate depletion. Impaired function of G proteins appears to be the most likely mechanism accounting for attenuated [3H]PI hydrolysis after chronic administration of lithium.     

Long-term phorbol ester treatment dissociates phospholipase D activation from phosphoinositide hydrolysis and prostacyclin synthesis in endothelial cells stimulated with bradykinin

Bovine pulmonary artery endothelial cells (BPAEC) were prelabeled with [3H]choline or [3H]myristic acid to selectively label endogenous phosphatidylcholine. BPAEC were stimulated with ATP and bradykinin (BK), and phospholipase D (PLD) activation was detected as a 4-fold increase in [3H]choline in cells prelabeled with [3H]choline or as a 2- to 3-fold increase in [3H]phosphatidylethanol in cells prelabeled with [3H]myristic acid and stimulated in the presence of ethanol. Pretreatment of BPAEC with 0.1 microM phorbol 12-myristate 13-acetate (PMA) for 22 hr completely inhibited agonist-induced PLD activation, whereas prostacyclin synthesis and [3H]phosphoinositide ([3H]PIns) hydrolysis were enhanced in pretreated cells. Long-term PMA treatment thus dissociates agonist-induced PLD activation from [3H]PIns hydrolysis, and agonist-induced prostacyclin synthesis is not dependent upon PLD activation.     

An alkylating derivative of oxotremorine interacts irreversibly with the muscarinic receptor

A 2-chloroethylamine derivative of oxotremorine was studied in pharmacological experiments and muscarinic receptor binding assays. The compound, N-[4-(2-chloroethylmethylamino)-2-butynyl]-2-pyrrolidone (BM 123), forms an aziridinium ion in aqueous solution at neutral pH that stimulates contractions of the guinea pig ileum with a potency similar to that of oxotremorine. Following the initial stimulation, there is a long lasting period of lack of sensitivity of the guinea pig ileum to muscarinic agonists. BM 123 also produces muscarinic effects in vivo. When homogenates of the rat cerebral cortex were incubated with BM 123 and assayed subsequently in muscarinic receptor binding assays, a loss of binding capacity for the muscarinic antagonist, [3H]N-methylscopolamine ( [3H]NMS), was noted without a change in affinity. Similar observations were made in [3H]1-3-quinuclidinyl benzilate ( [3H]1-QNB) binding assays on the forebrains of mice that had been injected with BM 123 24 hr earlier. The loss in receptor capacity for both [3H]NMS and [3H]1-QNB was prevented by atropine treatment. Kinetic studies of the interaction of BM 123 with homogenates of the rat cerebral cortex in vitro showed that the half-time for the loss of [3H]1-QNB binding sites increased from 10 to 45 min as the concentration of BM 123 decreased from 10 to 1 microM. In contrast to the aziridinium ion, the parent 2-chloroethylamine compound and the alcoholic hydrolysis product were largely devoid of pharmacological and binding activity.     

Synthesis and hydrolysis of cholesteryl esters by isolated rat-liver lysosomes and cell-free extracts of human lung fibroblasts

The objective of this study was to examine and characterize the cholesteryl ester synthesizing [S] and hydrolyzing [H] properties of the acid cholesteryl ester hydrolase (acid cholesteryl ester hydrolase), both in isolated rat liver lysosomes and in cell-free extracts from cultured fibroblasts. For both liver lysosomes and fibroblasts extracts, the major synthesizing activity was found around pH 4 and did not require exogenous ATP. The rate of hydrolysis was measured at pH 4.5. Several different inhibitors were used in order to characterize the reactions. Ammonium chloride did not markedly affect the activity of acid cholesteryl ester hydrolase at pH 4 [S] or 4.5 [H], whereas chloroquine was a potent inhibitor of acid CEase in both liver lysosomes and fibroblast extracts. The [S] activity of the acid cholesteryl ester hydrolase in either material was not affected by the acylCoA:cholesterol acyltransferase inhibitor Compound 58-035 from Sandoz. Progesterone, on the other hand, which is an often used acylCoA:cholesterol acyltransferase inhibitor, markedly blocked both activities of the acid CEase. Our results indicate that the lysosomal compartment of both studied tissues, in addition to hydrolysis activity, also have a significant esterification activity. It appears that both activities are carried out by the same enzyme.     

Synthesis, characterization and properties of uridine 5′-(α-d-apio-d-furanosyl pyrophosphate)

1. A method was developed for synthesizing UDP-apiose [uridine 5'-(alpha-d-apio-d-furanosyl pyrophosphate)] from UDP-glucuronic acid [uridine 5'-(alpha-d-glucopyranosyluronic acid pyrophosphate)] in 62% yield with the enzyme UDP-glucuronic acid cyclase. 2. UDP-apiose had the same mobility as uridine 5'-(alpha-d-xylopyranosyl pyrophosphate) when chromatographed on paper and when subjected to paper electrophoresis at pH5.8. When [(3)H]UDP-[U-(14)C]glucuronic acid was used as the substrate for UDP-glucuronic acid cyclase, the (3)H/(14)C ratio in the reaction product was that expected if d-apiose remained attached to the uridine. In separate experiments doubly labelled reaction product was: (a) hydrolysed at pH2 and 100 degrees C for 15min; (b) degraded at pH8.0 and 100 degrees C for 3min; (c) used as a substrate in the enzymic synthesis of [(14)C]apiin. In each type of experiment the reaction products were isolated and identified and were found to be those expected if [(3)H]UDP-[U-(14)C]apiose was the starting compound. 3. Chemical characterization established that the product containing d-[U-(14)C]apiose and phosphate formed on alkaline degradation of UDP-[U-(14)C]apiose was alpha-d-[U-(14)C]apio-d-furanosyl 1:2-cyclic phosphate. 4. Chemical characterization also established that the product containing d-[U-(14)C]apiose and phosphate formed on acid hydrolysis of alpha-d-[U-(14)C]apio-d-furanosyl 1:2-cyclic phosphate was d-[U-(14)C]apiose 2-phosphate. 5. The half-life periods for the degradation of UDP-[U-(14)C]apiose to alpha-d-[U-(14)C]apio-d-furanosyl 1:2-cyclic phosphate and UMP at pH8.0 and 80 degrees C, at pH8.0 and 25 degrees C and at pH8.0 and 4 degrees C were 31.6s, 97.2min and 16.5h respectively. The half-life period for the hydrolysis of UDP-[U-(14)C]-apiose to d-[U-(14)C]apiose and UDP at pH3.0 and 40 degrees C was 4.67min. After 20 days at pH6.2-6.6 and 4 degrees C, 17% of the starting UDP-[U-(14)C]apiose was degraded to alpha-d-[U-(14)C]apio-d-furanosyl 1:2-cyclic phosphate and UMP and 23% was hydrolysed to d-[U-(14)C]apiose and UDP. After 120 days at pH6.4 and -20 degrees C 2% of the starting UDP-[U-(14)C]apiose was degraded and 4% was hydrolysed.     

Identification of the phosphorylated intermediate of the Neurospora plasma membrane H+-ATPase as beta-aspartyl phosphate

The chemical nature of the phosphoryl enzyme linkage of the electrogenic proton-translocating ATPase (ATP phosphohydrolase, EC 3.6.1.3) in the plasma membrane of Neurospora has been identified as a mixed anhydride between phosphate and the beta-carboxyl group of an aspartic acid residue in the polypeptide chain. Incubation of isolated Neurospora plasma membrane vesicles containing 32P-labeled ATPase in buffers of increasing pH followed by analysis of the hydrolysis products yielded a pH versus hydrolysis profile characteristic of an acyl phosphate linkage. Reaction of labeled membranes with hydroxylamine at pH 5.3 also released [32P]i from the ATPase. Amino acid analyses of the Na[3H]BH4 reduction products obtained from membranes containing phosphorylated and dephosphorylated ATPase identified [3H]homoserine, the expected reduction product of beta-aspartyl phosphate, as the only additional tritiated reduction product in the samples from phosphorylated membranes. Tritium was not found in alpha-amino-delta-hydroxyvaleric acid, the reduction product of gamma-glutamyl phosphate, nor in proline, the degradation product of alpha-amino-delta-hydroxyvaleric acid. These results indicate that the phosphorylated intermediate of the Neurospora plasma membrane ATPase is a beta-aspartyl phosphate identical with that already known to exist in the Na+:K+- and Ca2+-translocating ATPases of animal cell origin. A common model for the mechanisms of all 3 ion-translocating ATPases is presented.     

Evidence for sialidase hydrolyzing gangliosides GM2 and GM1 in rat liver plasma membrane

Rat liver plasma membrane removed sialic acid from mixed bovine brain gangliosides more efficiently than from sialyllactose and orosomucoid with an optimal pH of 4.5. When individual gangliosides, each labeled with [14C]sialic acid or [3H]sphingosine, were tested, not only GD1a and GM3 but also GM2 and GM1, both of which had been considered to resist mammalian sialidases, were desialylated. The products of GM2 and GM1 hydrolysis were identified as asialo-GM2 and asialo-GM1, respectively, by thin-layer chromatography.     

Activation of phospholipase A2 by endothelin in cultured vascular smooth muscle cells

The ability of endothelin to promote phospholipid hydrolysis has been studied in myo-[2-3H]-inositol-, [3H]-arachidonic acid- or methyl-[3H]choline chloride-prelabelled cultured vascular smooth muscle cells (VSMC) from rat and bovine thoracic aortae and human omental vessels. The biochemical responses to endothelin were comparable between the different VSMC isolates. Endothelin promoted the accumulation of glycerolphospho[3H]inositol and concomitant loss of [3H]-inositol label from phosphatidylinositol. Exposure of [3H]choline-labelled VSMC to endothelin resulted in a loss of radioactivity from phosphatidylcholine that was inversely parallelled by an increase in water-soluble [3H]-choline metabolites. In [3H]-arachidonic acid ([3H]-AA)-labelled VSMC, endothelin induced extracellular release of [3H]-AA which derived from both phosphatidylcholine and phosphatidylinositol. Half-maximally effective concentrations of endothelin for all these responses were approximately 2-7 nM and did not vary between VSMC types. Endothelin-induced release of [3H]-AA into VSMC medium-overlay was inhibited by quinacrine and nordihydroguaiaretic acid but not by neomycin or indomethacin. The data herein implicate activation of phospholipase A2 by endothelin with subsequent metabolism of arachidonic acid via the lipoxygenase pathway.     

3H]guanidinoethylmercaptosuccinic acid binding to tissue homogenates. Selective labeling of enkephalin convertase

[3H]Guanidinoethylmercaptosuccinic acid (GEMSA), a potent inhibitor of enkephalin convertase, binds to membrane and soluble fractions of tissue homogenates saturably and reversibly with a KD of 6 nM. Specific binding accounts for greater than 95% of total binding. The highest levels of [3H]GEMSA binding occur in the pituitary gland and the brain, with much lower levels in peripheral tissues. GEMSA, guanidinopropylsuccinic acid, 2-mercaptomethyl-3-guanidinothiopropionic acid, aminopropylmercaptosuccinic acid, [Leu] enkephalin-Arg, and [Met]enkephalin-Arg inhibit [3H] GEMSA binding to crude rat brain homogenates, to crude bovine pituitary homogenates, and to pure enkephalin convertase with equal potencies. Their Ki values against [3H]GEMSA binding are similar to their Ki values against enkephalin convertase activity. EDTA and 1,10-phenanthroline markedly inhibit both binding and enzymatic activity. The ratio of the Vmax for 5-dimethylaminonaphthalene-1-sulfonyl-Phe-Leu-Arg to the Bmax (maximal number of binding sites) for [3H]GEMSA is about 2,000 min-1 in both pure enzyme preparations and crude tissue homogenates. [3H] GEMSA binding activity is found only in fractions containing enkephalin convertase during enzyme purification from bovine pituitary by L-arginine affinity chromatography. These data confirm that [3H]GEMSA binds only to enkephalin convertase in crude homogenates under our assay conditions. CoCl2 activates enzyme activity without altering the Ki of GEMSA against enzymatic hydrolysis and weakly inhibits [3H] GEMSA binding by increasing the KD.     

Identification of a novel insulin-sensitive glycophospholipid from H35 hepatoma cells

This study identifies and partially characterizes an insulin-sensitive glycophospholipid in H35 hepatoma cells. The incorporation of [3H]glucosamine into cell lipids was investigated. A major labeled lipid was purified by sequential thin layer chromatography using first an acid followed by a basic solvent system. After hydrochloric acid hydrolysis and sugar analysis by thin layer chromatography, 80% of the radioactivity in the purified lipid was found to comigrate with glucosamine. H35 cells were prelabeled with [3H]glucosamine for either 4 or 24 h and treated with insulin causing a dose-dependent stimulation of turnover of the glycophospholipid which was detected within 1 min. The purified glycolipid was cleaved by nitrous acid deamination indicating that the glucosamine C-1 was linked to the lipid moiety through a glycosidic bond. [14C]Ethanolamine, [3H]inositol, and [3H]sorbitol were not incorporated into the purified glycolipid. The incorporation of various fatty acids into this glycolipid was also studied. [3H]Palmitate was found to be preferentially incorporated while myristic acid, stearic acid, oleic acid, linoleic acid, linolenic acid, and arachidonic acid were either not incorporated or incorporated less than 10% of palmitate. The purified glycolipid labeled with [3H]palmitate was cleaved by treatment with phospholipase A2 but was resistant to mild alkali hydrolysis suggesting the presence of a 1-hexadecyl,2-palmitoyl-glyceryl moiety in the purified lipid. Treatment of labeled glycophospholipid with phosphatidylinositol-specific phospholipase C from Staphylococcus aureus generated a compound migrating as 1-alkyl,2-acyl-glycerol and a polar head group with a size in the range from 800 to 3500. These findings coupled with the nitrous acid deamination demonstrate that glucosamine was covalently linked through a phosphodiester bond to the glyceryl moiety of the purified glycolipid. These findings suggest that insulin acts on this glycophospholipid by stimulating an insulin-sensitive phospholipase C. This unique glycophospholipid may play an important role in insulin action by serving as precursor of insulin-generated mediators.     

Effects of temperature on allosteric and catalytic properties of the cGMP-stimulated cyclic nucleotide phosphodiesterase from calf liver

We have investigated effects of temperature on the catalytic and allosteric properties of the cGMP-stimulated cyclic nucleotide phosphodiesterase from calf liver. Vmax for cAMP and cGMP increased as assay temperature increased from 5 to 45 degrees C. At substrate concentrations below Kmapp, however, hydrolysis increased as temperature decreased from 45 to 5 degrees C and was much greater at 5 degrees C than at 45 degrees C. As assay temperature decreased, Kmapp for cAMP and cGMP decreased. Hill coefficients for cAMP and cGMP were approximately 1.9 at 45 degrees C and 1.2-1.0 at 5 degrees C. cGMP stimulated hydrolysis of 0.5 microM [3H]cAMP at all assay temperatures. Although maximal activity stimulated by cGMP, like Vmax, was lowest at 5 degrees C, presumably because of the effect of temperature on catalytic activity, the apparent activation constant (K alpha app) for cGMP stimulation was lower at 5 degrees C than at 45 degrees C. Thus, affinity for both substrate and effector was increased at 5 degrees C, suggesting that low temperature promotes transitions of the cGMP-stimulated phosphodiesterase to a "high affinity" state. That cGMP stimulated cAMP hydrolysis at 5 degrees C suggests that temperature-induced transitions are incomplete and/or readily reversible. In assays at 30 degrees C competitive inhibitors, like substrates, induce allosteric transitions which result in enhanced hydrolysis of low substrate (1.0 microM [3H] cAMP) concentrations. At higher substrate concentrations (50 microM [3H]cAMP), with the enzyme in the "activated" state, inhibitors compete with substrate at catalytic sites and reduce hydrolysis. At 45 degrees C, as at 30 degrees C, 1-methyl-3-isobutylxanthine (IBMX) and papaverine increased hydrolysis of 1.0 microM [3H]cAMP and reduced hydrolysis of 50 microM [3H]cAMP. At 5 degrees C, however, IBMX and papaverine inhibited hydrolysis of both 1.0 and 50 microM [3H]cAMP. Enzyme activity was relatively more sensitive to inhibition by IBMX at 5 degrees C than at 45 degrees C. Taken together, these observations support the notion that low temperature induces incomplete or readily reversible transitions to the high affinity state for substrates, effectors, and inhibitors. These observed effects of temperature also point out that enzyme determinants and topographical features responsible for transitions to the high affinity state and expression of catalytic activity can be regulated independently.     

Partial characterization of a major gut thiol proteinase from larvae of Callosobruchus maculatus F.

Much of the proteolytic activity in the digestive tract of Callosobruchus maculatus larvae can be attributed to a thiol proteinase(s) that hydrolyzes [3H]methemoglobin optimally at pH 5.0. Maximal hydrolysis of [3H]methemoglobin, [3H]alpha-casein, and N-benzoyl-DL-arginine napthylamide-(BANA) required the presence of thiol reducing agents. Larval gut proteinase activity was strongly inhibited by p-hydroxymercuribenzoic acid (pHMB), Nethylmaleimide (NEM), and iodoacetic acid (IAA) but was unaffected by the Bowman-Birk and Kunitz proteinase inhibitors from soybeans or by lima bean trypsin inhibitor. L-Trans-epoxysuccinyl-leucylamido-(4-guanidino)-butane (E-64), a specific inhibitor of thiol proteinases, potently inhibited proteolysis of [3H]methemoglobin by larval gut homogenates. Proteolytic activity in the larval gut was located in the lumen contents and thus appears to play a major role in extracellular digestion. The pH of the larval midgut is slightly acidic, and midgut contents exhibit a negative redox potential, conditions supporting the activity of a thiol proteinase. The significance of these findings is discussed with reference to the vulnerability of this digestive proteinase as a target for existing or genetically engineered plant chemical defenses.     

Insulin stimulates the generation of two putative insulin mediators, inositol-glycan and diacylglycerol in BC3H-1 myocytes

Recent evidence suggests that insulin induces hydrolysis of phosphatidylinositol-glycan (PI-G) and releases inositol-glycan (IG) and diacylglycerol (DAG). These two mediators are speculated to mediate different insulin actions. In this study, we examined metabolic labeling of PI-G in BC3H-1 myocytes with known precursors of PI-G. PI-G was metabolically labeled with [3H]myo-inositol, [3H]glucosamine, [3H]galactose, [3H]glycerol, and [3H]myristic acid. The treatment of 3H-labeled PI-G with phosphatidylinositol-specific phospholipase C liberated [3H]myo-inositol, [3H]glucosamine, or [3H]galactosamine-labeled IgGs, and [3H]glycerol or [3H]myristic acid-labeled DAG. In BC3H-1 myocytes, insulin induced phosphodiesteratic hydrolysis of PI-G and stimulated generation of IGs and DAG. Released IGs were labeled with [3H]myo-inositol, [3H]glucosamine, and [3H]galactose. Released DAG was labeled with [3H] glycerol and [3H]myristic acid. The IG had a dose-dependent insulin-like activity on glucose oxidation and lipogenesis without affecting glucose transport in rat adipocytes. Insulin increased 3H radioactivities of IG and insulin-mimicking activities of IG. These results provided further evidence that hydrolysis of PI-G and generation of IGs and DAG might be early steps in some insulin actions.