The kinetics of Ins(1,4)P2 dephosphorylation by Ins(1,4)P2 1-phosphatase in bovine brain.

Ins(1,4)P2 1-phosphatase catalyses the dephosphorylation of Ins(1,4)P2 to Ins(4)P. This enzyme was purified 3000-fold to a specific activity of 10-20 mumol/min/mg protein. Ins(1,4,5)P3 (0.04-1 microM) was not a substrate of the enzyme under conditions where 50% of Ins(1,4)P2 was dephosphorylated. All kinetics of Ins(1,4)P2 1-phosphatase displayed Michaelis-Menten behaviour. Both reaction products, Ins(4)P and phosphate inhibited the enzyme: Ins(4)P was a non-competitive inhibitor (Ki = 59 microM) and phosphate was competitive (Ki = 0.53 mM) with respect to Ins(1,4)P2 as substrate. In contrast, Li+ inhibition was uncompetitive (Ki at 1 mM LiCl was 2.7 mM).