Dehydrodolichyl diphosphate synthase in Sertoli and spermatogenic cells of prepuberal rats

Age-dependent changes in rat liver prenyltransferases

Mevalonate pathway lipids including cholesterol, ubiquinone and dolichol, are of great importance for cellular function. Many of the enzymes of this pathway are thus strictly regulated. During development of the rat, the cellular levels of certain of these lipids vary. Prenyltransferases have been investigated and it is reported here that farnesyl pyrophosphate synthase activity in rat liver cytosol decreases after birth to a lower, steady level. This decrease is not paralleled by the level of synthase protein, which shows two maxima, one immediately after birth and the other 30 days later. cis-Prenyltransferase activity is low after birth, increases continuously up to day-54 and then decreases to a low level which was maintained throughout the remainder of the study (365 days). Squalene synthase exhibits high activity after birth, but decreases during the first 100 days thereafter, and subsequently remains at the low level thus reached. In contrast to these changes in the activities of the prenyltransferases, the level of cholesterol is constant and the dolichol concentration increases continuously throughout the entire period studied.

Glycoprotein biosynthesis by testes of 40-day-old rats subjected to protein malnutrition

The testes of 40-day-old rats subjected to protein malnutrition show a marked delay in maturation of the seminiferous epithelium, as well as greater mannose incorporation into glycoprotein than observed in normal animals of the same age. Testes were incubated for 1 h with [2-3H]mannose and germ cells were then separated by the Staput method. Mannose incorporation occurred in the same cell fraction, i.e. the spermatocytes, both in normally fed and protein-undernourished animals. These data were confirmed by incubating the cells previously isolated on the gradient with [2-3H]mannose. Comparison of these data with results obtained in previous studies on 20-day-old animals in which mannose incorporation was lower in undernourished rats suggests that the differences observed in the present study between the experimental groups are due to alterations in the germ cells.

Control of N-linked oligosaccharide synthesis: cellular levels of dolichyl phosphate are not the only regulatory factor

When MDCK cells were incubated in the presence of the protein synthesis inhibitor puromycin or cycloheximide, there was a rapid and concentration-dependent inhibition in the incorporation of [2-3H]mannose into lipid-linked oligosaccharide and into protein. However, mannose incorporation into dolichyl-P-mannose was not affected. Interestingly, these inhibitors did block [6-3H]glucosamine incorporation into dolichyl-PP-GlcNAc as well as into lipid-linked oligosaccharides. Similar results were obtained when other cell lines were used and also when inhibitors of protein glycosylation such as beta-hydroxynorvaline and beta-fluoroasparagine were used. Cells incubated in puromycin did not show any changes in the levels of sugar nucleotides, GDP-mannose or UDP-GlcNAc, or in the in vitro activities of the glycosyltransferases that add mannose to the lipid-linked oligosaccharides. The inhibition of mannose incorporation into lipid-linked oligosaccharides could not be overcome by addition of dolichyl-P to the inhibited cells, even though the addition of dolichyl-P to control cells stimulated mannose incorporation into dolichyl-P-mannose, lipid-linked oligosaccharides, and protein from 3- to 5-fold. Thus, limitations in the levels of dolichyl-P do not appear to be a major factor in this inhibition. On the other hand, addition of the tripeptide acceptor N-acyl-Asn-Try-Thr did overcome the puromycin inhibition to some extent, suggesting that accumulation of some intermediate such as lipid-linked oligosaccharides might be involved in the inhibition.