Phosphofructokinase of calf trabecular meshwork.
The activity of phosphofructokinase (PFK), a key regulatory enzyme of glycolysis, has been measured in the 27,000 X g supernatant of homogenates prepared from excised calf trabecular meshwork. The enzyme required NH4+, both at pH 8.5 and pH 7.2. This requirement was not relieved by K+ or AMP. At pH 7.2 and ATP levels of 0.1 to 2.5 mM, PFK was completely inactive in the absence of added AMP or NH4+. PFK was only weakly activated by 0.5 mM AMP or by 5 mM NH4+, but in the presence of both AMP and NH4+, PFK was highly active up to 1 mM ATP. At pH 8.5 and ATP levels of 0.1-12.5 mM, PFK was weakly active in the absence of added NH4+, with or without AMP. With the addition of 5 mM NH4+, PFK was highly active up to 2.5 mM ATP, while AMP was largely without effect. Concentrations of NH4+ as low as 0.03 mM stimulated PFK activity to 20% of maximal, yet the maximum was not reached until NH4+ levels were 10-30 mM. The activation of PFK by AMP and its inhibition by ATP is profoundly modified by pH. In contrast, the requirement for NH4+ is unaffected. This requirement suggests a regulatory role for ammonium ion in controlling the rate of glycolysis in trabecular meshwork. The concentration of ammonium in calf aqueous humor was found to be 0.18 mM, which is in the right range to have an effect.
Anderson, PJ; Karageuzian, LN; Epstein, DL
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