Kinetics of the interaction of hemin liposomes with heme binding proteins

As a model for the transport of hemin across biological membranes, sonicated phosphatidylcholine liposomes with incorporated hemin were characterized. The interaction of the hemin liposomes with the heme binding proteins albumin, apomyoglobin, and hemopexin was examined as a function of liposome charge and cholesterol content. In all cases, there was an almost complete transfer of hemin from liposome to protein; a rapid phase and a slow phase were observed for the transfer. For negatively charged liposomes (with 11% dicetyl phosphate), the rapid and slow phases showed observed rates of transfer of ca. 2 and 0.01 s-1, respectively, for all three proteins. The presence of cholesterol in the liposomes decreased the observed rates by a factor of 2, and positively charged liposomes (with 11% stearylamine) showed about one-fifth the observed rates of negatively charged liposomes. The observed rates were independent of protein concentration, indicating that the rate-determining step is hemin efflux from the lipid bilayer. The hemin interaction with the phospholipid bilayer is suggested to be primarily hydrophobic with some electrostatic character. The two phases are suggested to arise from two different populations of hemin within the liposomes and are interpreted as arising from two different orientations of hemin within the bilayer.     

Phospholipid-Sensitive Ca2+-Dependent Protein Kinase Preferentially Phosphorylates Serine-115 of Bovine Myelin Basic Protein

Phospholipid-sensitive Ca2+ -dependent protein kinase (PL-Ca-PK) and cyclic AMP-dependent protein kinase (A-PK) both preferentially phosphorylated serine residues of bovine myelin basic protein (MBP). Tryptic peptide maps of MBP phosphorylated by PL-Ca-PK or A-PK, however, revealed different phosphopeptides, suggesting a difference in the intramolecular substrate specificity for the two enzymes. Serine-115 of MBP, in the sequence (-Arg-Phe-Ser(115)-Trp-), was found to be a preferred and probably major phosphorylation site for PL-Ca-PK. Because serine-115 of bovine MBP corresponds to serine-113 of rabbit MBP, an in vivo phosphorylation site reported by Martenson et al. (1983), and PL-Ca-PK is present at a very high level in brain and myelin, it is suggested that the enzyme may be responsible for the in vivo phosphorylation of this and other sites in MBP.     

Free-radical chain oxidation of 2-nitropropane initiated and propagated by superoxide.

The superoxide radical O2.-, whether produced by the xanthine/xanthine oxidase reaction or infused as KO2, solubilized by a crown ether in dry dimethyl sulphoxide, initiated a free-radical chain oxidation of anionic 2-nitropropane. Superoxide dismutase, but not catalase, inhibited oxidation of the nitroalkane. Xanthine oxidase suffered a syncatalytic inactivation, during the co-oxidation of 2-nitropropane, which was reversed by dialysis. Cyanide exacerbated this syncatalytic inactivation and rendered it irreversible. The frequently observed oxidations of nitroalkanes by flavoenzymes now need to be re-examined to clarify the extent to which O2.--initiated free-radical chain oxidation contributed to the overall nitroalkane oxidation.     

Metabolic heterogeneity of the proximal and distal kidney tubules

Proximal and distal tubule suspensions were prepared from kidneys of Sprague-Dawley rats by an isolation procedure on a Percoll^R gradient. The marker enzymes alkaline phosphatase (brush border) and hexokinase (cytoplasmic) as well as p-aminohippurate transport capacity, gluconeogenic activity and electron microscopy were used to characterize the two kidney tubule suspensions. The results of this study indicate that cytochrome P-450 is localized to the proximal tubular cells and that the O-deethylation of 7- ethoxycoumarin was higher in the proximal than distal fraction. Both proximal and distal tubules showed glucuronidation and deacetylation capacities and a relatively equal distribution of non-protein sulfhydryls. These studies demonstrate metabolic heterogeneity of the nephron, the proximal tubule being the main site of renal xenobiotic metabolism. Understanding of metabolic heterogeneity of proximal and distal kidney tubules should provide important information regarding cell specific mechanisms of nephrotoxicity.     

Nitrate reductase-deficient mutants in barley : immunoelectrophoretic characterization

Nitrate reductase-deficient barley (Hordeum vulgare L.) mutants were assayed for the presence of a functional molybdenum cofactor determined from the activity of the molybdoenzyme, xanthine dehydrogenase, and for nitrate reductase-associated activities. Rocket immunoelectrophoresis was used to detect nitrate reductase cross-reacting material in the mutants. The cross-reacting material levels of the mutants ranged from 8 to 136% of the wild type and were correlated with their nitrate reductase-associated activities, except for nar 1c, which lacked all associated nitrate reductase activities but had 38% of the wild-type cross-reacting material. The cross-reacting material of two nar 1 mutants, as well as nar 2a, Xno 18, Xno 19, and Xno 29, exhibited rocket immunoprecipitates that were similar to the wild-type enzyme indicating structural homology between the mutant and wild-type nitrate reductase proteins. The cross-reacting materials of the seven remaining nar 1 alleles formed rockets only in the presence of purified wild-type nitrate reductase, suggesting structural modifications of the mutant cross-reacting materials. All nar 1 alleles and Xno 29 had xanthine dehydrogenase activity indicating the presence of functional molybdenum cofactors. These results suggest that nar 1 is the structural gene for nitrate reductase. Mutants nar 2a, Xno 18, and Xno 19 lacked xanthine dehydrogenase activity and are considered to be molybdenum cofactor deficient mutants. Cross-reacting material was not detected in uninduced wild-type or mutant extracts, suggesting that nitrate reductase is synthesized de novo in response to nitrate.     

THE NACREOUS WALLS OF SIEVE ELEMENTS IN SEAGRASSES

The nacreous walls of sieve elements occur in seagrasses in all three genera of the family Zosteraceae and the genus Halodule of the family Cymodoceaceae but are absent from another eight seagrass genera belonging to the families Hydrocharitaceae, Cymodoceaceae, and Posidoniaceae. They occur in leaf blades, leaf sheaths, rhizomes, and erect stems but are not present in root tissues. The nacreous wall is uneven along the inner limits reflecting irregular thickness. The wall consists of hemicellulose or pectin and cellulose, but no protein, lignin, or lipid. Ultrastructurally, the wall contains parallel microfibrils or loose fibrils embedded in an amorphous matrix. Open pores occur in sieve plates and branching plasmodesmata are present in enlarged sieve areas. Mitochondria, endoplasmic reticulum, and plastids are also present in these sieve elements.     

Phospholipid-sensitive Ca2+-dependent protein kinase and its substrates in human neutrophils

Phospholipid-sensitive Ca²⁺-dependent protein kinase (PL-Ca-PK) was found to be present at a high level in human neutrophils, with its activity localized in the particulate fraction. In contrast, cyclic AMP-dependent protein kinase (A-PK) and cyclic GMP-dependent protein kinase (G-PK), present at lower levels compared to PL-Ca-PK, were localized in the cytosolic fraction. Phosphorylation of several endogenous proteins (mol. wts. 89,000, 38,000, 34,000, 17,000 and 15,000), also localized in the particulate fraction, was stimulated specifically by a combination of phosphatidylserine and Ca²⁺, whereas no substrate proteins were observed for the calmodulin-sensitive Ca²⁺-dependent protein kinase system under the same incubation conditions. Although no substrate proteins for G-PK were detected, one substrate (mol. wt. 19,000) for A-PK was observed. Phosphorylation of substrates for PL-Ca-PK, but not that for A-PK and for enzymes independent of Ca²⁺ or cyclic AMP, was inhibited by a variety of agents, including trifluoperazine, W-7 [N-(6-aminohexyl)-5-chloro-1-naphthalene-sulfonamide], adriamycin, palmitoylcarnitine, and melittin. The present findings suggest that the $\text{phospholipid}\text{Ca}{}^{\mathrm{2+}}\text{-stimulated}$ protein phosphorylation system may be important in the membrane associated functions of human neutrophils.     

The corrected nucleotide sequence of yeast leucine transfer ribonucleic acid

The nucleotide sequence of “Renaturable” leucine transfer RNA from Baker's yeast has been re-investigated. The results showed that (i) this tRNA has a sequence of DCD at positions 19–21, (ii) it has an anticodon m⁵CAA and (iii) it has a pseudouridine at position 40.