A study on the effect of lysolecithin and phospholipase A on membrane-bound galactosyltransferase

Addition of lysolecithin caused very marked activation of UDP-galactose:glycoprotein galactosyltransferase in rat liver microsomes and in Golgi-rich membranes. Lysolecithin activated galactosyltransferase when the enzyme was assayed both with endogenous acceptor and with exogenous proteins or monosaccharides as acceptors. Lactose synthetase activity in presence of α-lactalbumin was also stimulated by lysolecithin. Lecithin, lysophosphatidylethanolamine, lysophosphatidic acid, and glycerophosphorylcholine did not activate the enzyme, suggesting that both fatty acyl and phosphorylcholine groups of the lysolecithin molecule are required for the observed activation. The degree of activation was about the same when myristoyl-, palmitoyl-, oleoyl-, or stearoyllysolecithin were tested. The activation by lysolecithin was observed well within the physiological concentration of the lipid in the liver cell. Saturating amounts of Triton masked the effect of lysolecithin.Brief preincubation with phospholipase A activated the enzyme and generated lysolecithin in the membranes. Triton and lysolecithin activated the enzyme without any lag time, whereas phospholipase A activation was dependent on preincubation and also on an alkaline pH favorable for the hydrolysis of phospholipid. EDTA blocked the activation effect of phospholipase A but had no effect on activation by lysolecithin. Albumin and cholesterol opposed the effects of lysolecithin and phospholipase A on the enzyme. Two successive incubations of the microsomes with lysolecithin caused considerable release of the enzyme into the soluble fraction. The role of lysolecithin in the activation of the enzyme is probably related to the solubilization of the membrane and consequent enhanced interaction of the enzyme with substrate. Lysolecithin also activated N-acetylglucosaminyl- and sialyltransferase activities in microsomes. A possible role of lysolecithin is indicated in the regulation of glycosylation reactions in mammalian system.

Activities of prolyl hydroxylase, lysyl hydroxylase, collagen galactosyltransferase and collagen glucosyltransferase in the liver of rats with hepatic injury

The activities of four enzymes catalysing post-translational modifications of the collagen polypeptide chains were assayed in the livers of rats with experimental hepatic injury. The liver injury was induced by injecting carbon tetrachloride twice weekly, and assays of the enzymic activities were carried out 2 and 4 weeks after commencement of administration of carbon tetrachloride. The liver homogenates were preincubated with Triton X-100 before the assays, because such treatment was found to increase the activities of all four enzymes in the supernatants of liver homogenates. The activities of all four enzymes had increased by 2 weeks after commencement of carbon tetrachloride administration. No increase was found in the collagen content of the livers at this stage and thus an increase in all four enzyme activities preceded an increase in the collagen content of the liver. A further slight increase was found in three of the enzyme activities during the subsequent 2 weeks of the experiment, whereas no further increase was found in the collagen galactosyltransferase activity. A statistically significant correlation was found between all four enzyme activities, but the magnitude of the increases varied considerably. The largest increase was found in lysyl hydroxylase activity, and at 4 weeks the magnitude of this was about three times that of the collagen galactosyltransferase activity. The results thus indicate that the increased enzyme activities cannot be explained simply by an increase in the number of collagen-producing cells having similar enzyme activity patterns to those of the cells initially present in the liver.

Selective activation of mouse T and B lymphocytes by periodate, galactose oxidase and soybean agglutinin

Modifications of surface saccharides of mouse spleen cells wirh periodate (NaIO4) or with neuraminidase and galactose oxidase induced extensive blastogenesis. The mitogenic effects of these agents on purified thymus‐derived lymphocytes (T cells) and bone marrow‐derived lymphocytes (B cells) were investigated. Thymus cells were stimulated after treatment with NaIO4 or with neuraminidase and galactose oxidase. The stimulation of these cells was markedly enhanced upon addition of a lymphocyte‐activating factor obtained from human leukocyte culture. Spleen cells from congenitally athymic mice did not respond to treatment with NaIO4 or with neuraminidase and galactose oxidase. Spleen cells from mice deprived of T cells by thymectomy, irradiation and bone marrow reconstitution, responded poorly to the above mentioned agents. It was concluded that periodate or treatment with neuraminidase and galactose oxidase stimulates exclusively T cells. Soybean agglutinin (SBA) had selective effects on T and B mouse lymphocytes. Neuraminidase treatment rendered T cells highly responsive to stimulation by SBA. On the other hand the small but significant stimulation of B cells by SBA is reduced after treatment of the cells with neuraminidase. SBA stimulates B cells and neuraminidase‐treated T cells by interaction with galactose‐like sites.