Enzymatic regulation of glycoprotein synthesis in peripheral nervous system myelin

The enzyme UDP-N-acetylglucosamine: dolichyl phosphate, N-acetylglucosamine-1-phosphate transferase initiates the synthesis of the oligosaccharide chain of complex-type glycoproteins. In view of the high content of glycoprotein in peripheral nerve myelin, the properties of this enzyme, its changes with age, and the effect of the specific inhibitor tunicamycin were investigated. The enzyme activity in rat peripheral nerve homogenate was completely dependent on the presence of exogenous dolichyl phosphate as well as Mg2+ and a detergent (Triton X-100) and was also greatly stimulated by a high salt concentration (0.4 M KCl) and AMP. The highest specific activity was present in the postmitochondrial membranes. The specific activity in postmitochondrial membranes in the presence of exogenous dolichyl phosphate reached a maximum at 17 days and remained relatively high throughout development, up to 2 years of age, but the activity was much lower when dolichyl phosphate was not added. This indicates that the enzyme level does not decrease with age, but that the content of the lipid cofactor may limit glycoprotein synthesis in vivo. Tunicamycin (5 micrograms) was injected intraneurally into 24-day-old rat sciatic nerve, and the enzyme was assayed from 1 to 24 days after injection. The specific activity of the transferase remained at low levels (5-40% of the level in control nerve) in most injected nerves assayed throughout this postinjection period. A protein previously identified as the unglycosylated P0 protein was synthesized in vitro by the tunicamycin-injected nerve and could be demonstrated to be incorporated into myelin in large amounts at 2 days and in small amounts at 6 days after injection.(ABSTRACT TRUNCATED AT 250 WORDS)