Kinetic properties of d-fructose-1,6-bisphosphate 1-phosphohydrolase isolated from human muscle

Abstract

D-Fructose-1,6-bisphosphate 1-phosphohydrolase (EC 3.1.3.11) [Fru(1,6)Pase] was isolated from human muscle in an electrophoretically homogeneous form, free of aldolase contamination. The enzyme is inhibited by the substrate [fructose (1,6)-bisphosphate]. Km is 0.77 microM; Kis is 90 microM. The fructose-2,6-bisphosphate [Fru(2,6)P2], a regulator of gluconeogenesis, inhibits human muscle Fru(1,6)Pase with Ki = 0.13 microM. To determine Km, Kis and Ki the integrated method was used. AMP is an allosteric inhibitor of Fru(1,6)Pase. As with other mammalian isoenzymes, the human muscle enzyme is more strongly inhibited by AMP than is the liver isoenzyme [Dzugaj and Kochman (1980) Biochim. Biophys. Acta 614, 407-412]. Both of the inhibitors [AMP and Fru(2,6)P2] act synergistically on human muscle Fru(1,6)Pase. Ki for Fru(2,6)P2 determined in the presence of 0.4 microM AMP was 0.028 microM. The human muscle enzyme, like other mammalian Fru(1,6)Pases, requires Mg2+ for its activity. The Ka for magnesium was 232 microM, and h (Hill coefficient) = 2.0.