Neurons, chromaffin cells and membrane fusion

On the Occurrence of Multiple Isoprenylated Cysteine Methyl Ester Hydrolase Activities in Bovine Adrenal Medulla

Rab proteins intervene in the controlled exocytosis of catecholamines by chromaffin cells from the adrenal medulla. These proteins are posttranslationally modified by digeranylgeranylation and carboxymethylation. Reversible carboxymethylation terminating the isoprenylation pathway may play an important role in both the functioning and the subcellular housing of small G-proteins. Controlled methylation infers a rational interplay between the two enzymes involved i.e., the protein-S-prenylcysteine methyltransferase and the opposing esterase. Previously we have identified a methyltransferase type III in chromaffin cells. In this paper we focus on the corresponding demethylase. The methyl ester hydrolase activity was monitored using AFCM and AGGCM as artificial substrates while p-nitrophenylacetate was adopted as a pseudosubstrate for nonspecific esterase action. Based on subcellular fractionation experiments, kinetic studies and screening a battery of potential effectors, including a series of metallic ions and metal chelators, multiple sulphydryl reagents and host of specific protease/esterase inhibitors, it is suggested that at least two prenylcysteine carboxymethyl esterase isoenzymes are operational in bovine adrenal medulla. These isoenzymes are distinctly different from the nonspecific esterase.