Insignificance of Gluconeogenesis in Human Blood Platelets

Abstract

Human blood platelets contain no detectable activity of the enzymes fructose diphosphatase (EC 3.1.3.11), phospho-enolpyruvate carboxykinase (EC 4.1.1.32) and pyruvate carboxylase (EC 6.4.1.1.). Glucose-6-phosphatase (EC 3.1.3.9) activity is very low. Phosphofructokinase present in human blood platelets, catalyzes a reaction which can be stimulated by AMP in a platelet homogenate, due to the presence of endogenous ADP and myokinase. These enzymes are responsible for the formation of fructose-6-phosphate from fructose-1, 6-diphosphate. Pyruvate kinase (EC 2.7.1.40) in human blood platelets belongs to the M-type, which is not inhibited by ATP, at least not under the conditions applied. The results obtained indicate that gluconeogenesis in human blood platelets is not present in the way which has been established for liver and kidney.

Regulation of Carbohydrate Metabolism in Platelets

“…Incorporation of labeled pyruvate in glucose and glycogen is therefore probably the result of the backward reaction through the various equilibrium reactions outlined above and through the phosphofructokinase and hexokinase reactions which although irreversible within the intact cell can shift to the formation of fructose-6-P and glucose provided that high levels of fructose-1,6-P2 and glucose-6-Pare present. The high levels of pyruvate and A TP used in these studies favour the backward reactions especially when A TP is converted by adenylate kinase into ADP which is the second substrate for both enzymes for the backward reaction (Schrijver et al 1975). Synthesis of glucose and glycogen from pyruvate therefore probably does not take place.…”

“…1970c. But platelets lack the key enzymes of carbohydrate synthesis: P-enolpyruvate carboxy kinase, pyruvate carboxy kinase and Fru-1 ,6-P2-ase whereas Glu-6-P-ase activity is very low (Schrijver et al 1975, Soyama et al 1973, Czapek et al 1973. Incorporation of labeled pyruvate in glucose and glycogen is therefore probably the result of the backward reaction through the various equilibrium reactions outlined above and through the phosphofructokinase and hexokinase reactions which although irreversible within the intact cell can shift to the formation of fructose-6-P and glucose provided that high levels of fructose-1,6-P2 and glucose-6-Pare present.…”

Unreliability of platelet glucose‐6‐phosphatase for the diagnosis of glycogen storage disease type Ia

“…Stormont et al (1976) used the assay of glucose-1-phosphatase activity to measure non-specific activity. This may not be valid for platelet assay, as platelets contain a phosphoglucomutase that converts glucose 1phosphate to glucose 6-phosphate (Schrijver et al 1975). Shrijver et al (1975) suggested that fi-glycerolphosphate could be used as substrate for blocking non-specific phosphatase activity.…”