Evidence that glycosylation of pro-opiocortin and ACTH influences their proteolysis by trypsin and blood proteases

The role of the carbohydrate in the stabilization and protection of the glycoprotein, pro-opiocortin, from non-specific proteolysis by trypsin and blood proteases was studied in vitro. [3H]Arginine-labeled, glycosylated and non-glycosylated forms of pro-opiocortin were isolated from frog neurointermediate lobes and subjected to proteolysis by trypsin. The non-glycosylated form was degraded by trypsin more rapidly than the glycosylated form. Analysis of the tryptic products after trypsin treatment, showed that the non-glycosylated pro-opiocortin was cleaved to unidentified peptides within 1 min, whereas the glycosylated prohormone yielded 2 products, mol. wt. 23 000 ACTH and mol. wt. 21 000 ACTH, synthesized by the intact neurointermediate lobe. These data provide direct evidence in support of the hypothesis, derived from studies on the intact lobe (Loh and Gainer, 1978, 1979) that the glycosylation of pro-opiocortin is important: (1) to protect it against non-specific proteolysis in situ and (2) to direct processing by limiting proteolysis. In addition, we demonstrate that glycosylated forms of ACTH are much more stable in blood than non-glycosylated forms.