3'-PHOSPHOADENOSINE 5'-PHOSPHOSULFATE BIOSYNTHESIS AND THE SULFATION OF CHOLECYSTOKININ BY THE TYROSYLPROTEIN-SULFOTRANSFERASE IN RAT-BRAINTISSUE

This article resumes the work we have accomplished in the past few yea rs. Cholecystokinin sulfation is an important post-translational modif ication necessary for the biological activity of this peptide hormone. The tyrosyl protein sulfotransferase (TPST) activity from rat cerebra l cortex was characterized. TPST activity is most probably responsible for the endogenous sulfation of CCK. TPST reaction kinetic properties were studied using radiolabeled 3'-phosphoadenosine 5'-phosphosulfate (PAPS) and the non-sulfated :peptide acceptor terbutyloxycarbonyl-cho lecystokinin octapeptide (BocCCK-8(ns)) as substrates, and brain micro somes as the enzyme source. The BocCCK-8 sulfating reaction data is co nsistent with the idea that TPST forward reaction follows an ordered B i Bi mechanism. PAPS biosynthesis and availability was studied in slic es from rat cerebral cortex incubated in the presence of [S-35]sulfate . There is a rapid and dynamic turnover of the steady-state level of P APS in brain cells which is decreased by depolarizing agents such as p otassium, veratridine and glutamate. Furthermore, the presence of a me mbrane-bound PAPS biosynthesis inhibitor was observed. These results a re discussed in view of the biological importance that the cell sulfat ing pathways might play in nerve cell activity.