Interrelationship between glycolysis and the anaerobic part of the pentose phosphate pathway of carbohydrate metabolism in the myocardium

Abstract

The reactions of the anaerobic part of the pentose cycle of carbohydrate metabolism and glycolysis in the heart muscle have been compared. In heart muscle the transaldolase activity is either very low or altogether absent; therefore, erythrose 4-phosphate cannot be formed via the transaldolase reaction. The formation of heptulose 1,7-diphosphate from erythrose 4-phosphate and dihydroxyacetone phosphate occurs much faster than fructose 1,6-diphosphate synthesis from triose phosphates. The split-off inorganic phosphate from heptulose 1,7-diphosphate under the action of phosphatase occurs at a higher rate than that from fructose 1,6-diphosphate. The heptulose 7-phosphate formed is an immediate precursor of pentose phosphates in a transketolase reaction with glyceraldehyde 3-phosphate. Hence, during pentose phosphate synthesis from glycolytic products heptulose 1,7-diphosphate after being converted into heptulose 7-phosphate is utilized for the synthesis of pentose phosphates and does not serve as a pool for erythrose 4-phosphate. Under excess of pentose phosphates and the lack of glycolytic products the transketolase reaction between pentose 5-phosphate and erythrose 4-phosphate results in a formation of fructose 6-phosphate and glyceraldehyde 3-phosphate. Thus, erythrose 4-phosphate plays an essential role, i.e., under excess of glycolytic products this process can be channelled into the direction of pentose phosphate formation, while under excess of pentose phosphates — in the direction of synthesis of glycolytic products. The efficient formation of erythrose 4-phosphate occurs via the transketolase reaction between two initial products of glycosis, i.e., glucose 6-phosphate and fructose 6-phosphate to form erythrose 4-phosphate and octulose 8-phosphate. The hexokinase and phosphofructokinase reactions limit the rate of glycolysis; consequently, the retardation of early steps of glycolysis facilitates the participation of glucose 6-phosphate and fructose 6-phosphate in the transketolase reaction. The further pathways of erythrose 4-phosphate utilization are miscellaneous; some of them have been mentioned above. Octulose 8-phosphate can be involved in many reactions, particularly in the transketolase reaction with glyceraldehyde 3-phosphate, resulting in the formation of pentose phosphate and hexose 6-phosphate. Both compounds undergo subsequent conversions depending on the presence and concentration of substrates, which can further be utilized in the reactions of pentose cycle or glycolysis.