VITAMIN K-DEPENDENT CARBOXYLASE - EFFECT OF AMMONIUM-SULFATE ON SUBSTRATE CARBOXYLATION AND ON INHIBITION BY STEREOSPECIFIC SUBSTRATE-ANALOGS

(1) High concentrations of ammonium sulfate may stimulate the carboxylase activity of bovine liver microsomes about 10-fold. This effect results from an increase of the Vmax, whereas neither the apparent Km for a number of substrates nor the Ki for substrate analogs is affected. (2) The effect of ammonium sulfate was only found in substrates lacking the pro-sequence. No effect was measurable on the carboxylation of pro-PT28 and endogenous precursor proteins. (3) If the pro-fragment was added as a peptide not covalently bound to a carboxylatable substrate, the carboxylation thereof was only slightly affected and ammonium sulfate remained active as a stimulator of carboxylase activity. (4) S-MeTPT is a much stronger inhibitor of carboxylase activity than is R-MeTPT. (5) The inhibition of carboxylase by the methylated tripeptides is competitive and independent of the type of substrate. Also pro-PT28, which contains the full pro-sequence, could be inhibited completely. (6) On the other hand the carboxylation of endogenous protein precursors could only be partly inhibited by the substrate analogs: even at high concentrations of S-MeTPT a residual endogenous substrate carboxylation of about 30% was left. © 1990.