The functional significance of variants of fructose 1,6-diphosphatase in the gill and hypodermis of a marine crustacean. A kinetic study.

1. In the hypodermis and gill of the Crustacea fructose 1,6-diphosphatase (EC 3.1.3.11) functions at a primary branch point between glycogen and chitin synthesis. In these tissues of the Arctic king-crab, Paralithodes camtchatica, fructose diphosphatase occurs in two electrophoretically distinguishable forms. 2. Fructose diphosphatase I (pI7.2-7.5) accounts for 70 and 10% of total fructose diphosphatase activity in the hypodermis and gill respectively, whereas fructose diphosphatase II (pI5.3) accounts for 30 and 90% of the total activity in the two tissues. Both forms display a neutral pH optimum, have an absolute requirement for a bivalent cation, and are potently inhibited by high concentrations of AMP and substrate. 3. Fructose 1,6-diphosphate saturation follows Michaelis-Menten kinetics for both fructose diphosphatases; the K(m) (fructose diphosphate) for fructose diphosphatase I is somewhat higher than for fructose diphosphatase II. In the presence of 50-200mm-K(+), the K(m) (fructose diphosphate) increases and at high concentrations of K(+) fructose diphosphate saturation follows sigmoidal kinetics. 4. UDP-N-acetylglucosamine and UDP-glucose at high concentrations specifically and potently inhibit fructose diphosphatase II, but do not significantly affect fructose diphosphatase I activity. 5. Low concentrations of UDP-N-acetylglucosamine activate fructose diphosphatase II by a decrease in the apparent K(m) (fructose diphosphate), but fructose diphosphatase I is again refractory to UDP-N-acetylglucosamine under these conditions. 6. In the presence of K(+) and UDP-N-acetylglucosamine, fructose diphosphatase II is able to compete for limiting fructose diphosphate about three times more effectively than is fructose diphosphatase I. 7. AMP inhibition of both forms of the enzyme is subject to three independent variables: (a) alkaline pH increases the K(i) (AMP), (b) K(+) decreases the K(i), increases the sigmoidicity of inhibition kinetics, increases the maximum inhibition attained, and abolishes the effect of pH on AMP inhibition, and (c) Mg(2+) strongly de-inhibits AMP-inhibited fructose diphosphatase. 8. It is postulated that the presence of two forms of fructose diphosphatase aids controlled channelling of carbon through the fructose diphosphatase ;bottleneck' either towards glycogen synthesis or chitin synthesis, but not towards both simultaneously.