Pathways of glucose utilization in mammary tissue

Abstract

Although the most important method of glucose catabolism is undoubtedly by degradation via the classical Embden-Meyerhof-Parnas glycolytic pathway followed by oxidation of the triose by the tricarboxylic acid cycle, the existence of alternative routes of oxidation is now certain and these can, indeed, operate exclusively in some micro-organisms. Of these alternative pathways, the most important is that originally discovered by Warburg in yeast extracts and is now generally referred to as the pentose phosphate pathway. The pioneer work of Warburg, Lipmann and Dickens has been reviewed by Dickens (1953). Renewed interest in this pathway during the last few years has established its widespread occurrence in animal tissues, plants and micro-organisms. Precise evidence for its cyclic nature and characterization of the intermediates and enzyme reactions involved can be attributed chiefly to Horecker and Racker and their co-workers (Horecker & Mehler 1955; Gunsalas, Horecker & Wood 1955; Racker 1957). This somewhat complex cycle is shown in figure 31. Of the three glucose 6-phosphate molecules required for each turn of the cycle, two are regenerated from pentose phosphate by reactions catalyzed by transketolase and transaldolase. All of these enzyme reactions, like those of the glycolytic route, are localized essentially in the soluble fraction of cell homogenates (Glock & McLean 1954; Newburgh & Cheldelin 1956).