Comparative study of phosphofructokinase from rat small intestine and liver. Control by fructose-2,6-bisphosphate and other effectors.

As compared to the liver, intestinal mucosa shows a high rate of aerobic glycolysis. This difference has been attributed to the higher activity of the intestinal phosphofructokinase (PFK) isoenzyme. The regulatory properties of rat small intestine and liver PFK were investigated. At pH 8, where PFK activity can be evaluated free of allosteric influences, the specific activity of the liver isoenzyme was 25% higher than that of the intestinal one. At pH 7 the mucosal PFK was activated to 80% of its maximal activity at pH 8, while the liver enzyme showed only a 40% activation. The apparent Kms for Fructose-6-P were 0.47 and 1.03 mM for the mucosal and hepatic isoenzymes, respectively. At 2 mM Fructose-6-P, the optimal ATP concentration for both isoenzymes was 1 mM. Higher ATP concentrations strongly inhibited both enzymes, but, below 3 mM, PFK activity was larger in the mucosal homogenate. In addition, the intestinal PFK was more sensitive to activation by Fructose-2,6-bisphosphate and 6-phosphogluconate, particularly at low Fructose-6-P concentrations, and by AMP below 0.3 mM. These studies suggest that, under physiological conditions, the intestinal isoenzyme is more active than its liver counterpart. This may account for the high rate of aerobic glycolysis observed in the intestinal mucosa.