CHARACTERIZATION OF SERINE THREONINE PROTEIN PHOSPHATASES IN RINM5F INSULINOMA CELLS/

This study investigates the occurrence and regulation of serine/threon ine protein phosphatases (PPases) in insulin-secreting RINmSF insulino ma cells. PPases types 1 and 2A were identified in crude RINm5F cell h omogenates by both enzymatic assay and Western blot analysis. We then characterized and compared the inhibitory actions of several compounds isolated from cyanobacteria, marine dinoflagellates and marine sponge s, (viz. okadaic acid, microcystin-LR, calyculin-A and nodularin) cati on-independent PPase activities in RINm5F cell homogenates. It was fou nd that okadaic acid was the least potent inhibitor (IC(50)approximate to 10(-9) M, IC(110)approximate to 10(-6) M), while the other compoun ds exhibited IC50 values of approximate to 5.10(-10) M and IC(100)appr oximate to 5.10(-9) M. The findings indicate that the inhibitory subst ances employed in this study may be used pharmacologically to investig ate the role of serine/threonine PPases in RINm5F cell insulin secreti on, a process that is likely to be regulated to a major extent by prot ein phosphorylation.