1. Phytohaemagglutinin induced early changes in the catabolism of glucose by normal human lymphocytes suspended in a bicarbonate buffer. During 4hr. incubation glucose utilization was almost doubled. 2. The rates of several reactions in the metabolism of glucose were estimated. Total pyruvate formation, lactate production and fatty acid synthesis were stimulated to the same degree as was glucose utilization. The pentose cycle and the glycogen synthesis were also stimulated but less than was glucose utilization. The pentose cycle was found to account for 1·4% and 0·9% of the total glucose utilization without and with phytohaemagglutinin respectively. In these cells rates of triose phosphate iso-merization were at least six to seven times the rate of glucose phosphorylation. On an average 55–60% of the total carbon dioxide evolved was derived from decarboxylation of pyruvate, 25–30% from the tricarboxylic acid cycle and about 15% from the pentose cycle. Observed ratios of 14C specific yields in glycogen from [1−14C]- and [6−14C]-glucose could possibly be explained by assuming the existence of two separate glucose 6-phosphate pools. 3. During 4hr. incubation in bicarbonate buffer 14C from [U−14C]serine was incorporated into perchloric acid-insoluble material. This incorporation was stimulated by phytohaemagglutinin but was almost completely inhibited by puromycin. Puromycin also abolished phytohaemagglutinin-induced stimulation of glycolysis.

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