Zinc cation performed a role of activator as well as inhibitor for production of lactate from glucose 6-phosphate in the cytosol fraction of rat muscle. The pH optimum for glycolysis was 7.3 when Zn
2+ acted as an activator. At concentrations lower than 0.46mM, Zn
2+ was shown to be a more effective activator than Mg
2+ with an apparent
K1/2 of approximately 0.1mM. However, at concentrations higher than 0.5mM, Zn
2+ inhibited lactate production. The activatory as well as inhibitory effect of Zn
2+ on lactate production was investigated by the estimation of glycolytic intermediates. From the crossover plot, lactate production reflected phosphofructokinase activity, when Zn
2+ was used as a catalytic cation for both reactions. Phosphofructokinase activity in purified muscle was activated by Zn
2+ with an apparent
Km of approximately 0.05mM, but at high Zn
2+ concentrations, the enzyme activity was inhibited with an I
50 of 0.23mM in the presence of ATP. From these findings, it appears that lactate production might also depend on phosphofructokinase activity when Zn
2+ is used as an activating cation.
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