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Retinyl phosphate mannose synthesis in rat liver membranes. Phospholipase sensitivity and phospholipid requirement.

ABSTRACT: A remarkable and immediate decrease in GDP-mannose:retinyl phosphate mannosyltransferase activity was found on pre-incubation of rat liver postnuclear membranes with phospholipase A2 or phospholipase C. Under the same conditions of pre-incubation (1 min at 37 degrees C) trypsin did not affect the enzyme activity, whereas pre-incubation for 30 min with trypsin and Pronase abolished enzyme activity. The lipid extract of untreated rat liver membranes partially restored enzyme activity after phospholipase treatment. Sphingomyelin was as active as the endogenous lipids. Other phospholipids were less active in the following order: phosphatidylcholine greater than phosphatidylethanolamine greater than phosphatidylinositol = phosphatidylserine. Dolichyl phosphate mannose synthesis was inhibited less (33%) by phospholipase C than was Ret-P-Man synthesis (98.5%) under identical conditions of incubation, which included 0.025% Triton. However, retinyl phosphate mannose synthesis by purified endoplasmic reticulum was found to be resistant to phospholipase C. Mixing experiments failed to demonstrate an inhibitory effect of the phospholipase-treated postnuclear membrane fraction on the synthetic activity of the endoplasmic reticulum, thus excluding the release of an inhibitory factor from the postnuclear membranes.

SUBMITTER: Shidoji Y

PROVIDER: S-EPMC1152568 | biostudies-other | 1983 Dec

REPOSITORIES: biostudies-other

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Project description:In the absence of detergent, the transfer of mannose from GDP-mannose to rat liver microsomal vesicles was highly stimulated by exogenous retinyl phosphate in incubations containing bovine serum albumin, as measured in a filter binding assay. Under these conditions 65% of mannose 6-phosphatase activity was latent. The transfer process was linear with time up to 5min and with protein concentration up to 1.5mg/0.2ml. It was also temperature-dependent. The microsomal uptake of mannose was highly dependent on retinyl phosphate and was saturable against increasing amounts of retinyl phosphate, a concentration of 15mum giving half-maximal transfer. The uptake system was also saturated by increasing concentrations of GDP-mannose, with an apparent K(m) of 18mum. Neither exogenous dolichyl phosphate nor non-phosphorylated retinoids were active in this process in the absence of detergent. Phosphatidylethanolamine and synthetic dipalmitoylglycerophosphocholine were also without activity. Several water-soluble organic phosphates (1.5mm), such as phenyl phosphate, 4-nitrophenyl phosphate, phosphoserine and phosphocholine, did not inhibit the retinyl phosphate-stimulated mannosyl transfer to microsomes. This mannosyl-transfer activity was highest in microsomes and marginal in mitochondria, plasma and nuclear membranes. It was specific for mannose residues from GDP-mannose and did not occur with UDP-[(3)H]galactose, UDP- or GDP-[(14)C]glucose, UDP-N-acetyl[(14)C]-glucosamine and UDP-N-acetyl[(14)C]galactosamine, all at 24mum. The mannosyl transfer was inhibited 85% by 3mm-EDTA and 93% by 0.8mm-amphomycin. At 2min, 90% of the radioactivity retained on the filter could be extracted with chloroform/methanol (2:1, v/v) and mainly co-migrated with retinyl phosphate mannose by t.l.c. This mannolipid was shown to bind to immunoglobulin G fraction of anti-(vitamin A) serum and was displaced by a large excess of retinoic acid, thus confirming the presence of the beta-ionone ring in the mannolipid. The amount of retinyl phosphate mannose formed in the bovine serum albumin/retinyl phosphate incubation is about 100-fold greater than in incubations containing 0.5% Triton X-100. In contrast with the lack of activity as a mannosyl acceptor for exogenous dolichyl phosphate in the present assay system, endogenous dolichyl phosphate clearly functions as an acceptor. Moreover in the same incubations a mannolipid with chromatographic properties of retinyl phosphate mannose was also synthesized from endogenous lipid acceptor. The biosynthesis of this mannolipid (retinyl phosphate mannose) was optimal at MnCl(2) concentrations between 5 and 10mm and could not be detected below 0.6mm-MnCl(2), when synthesis of dolichyl phosphate mannose from endogenous dolichyl phosphate was about 80% of optimal synthesis. Under optimal conditions (5mm-MnCl(2)) endogenous retinyl phosphate mannose represented about 20% of dolichyl phosphate mannose at 15min of incubation at 37 degrees C.

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Retinyl phosphate mannose synthesis in rat liver membranes. Phospholipase sensitivity and phospholipid requirement.

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