1. The maximum activity of
hexokinase in lymphocytes is similar to that of
6-phosphofructokinase, but considerably greater than that of
phosphorylase, suggesting that
glucose rather than
glycogen is the major
carbohydrate fuel for these cells.
Starvation increased slightly the activities of some of the glycolytic
enzymes. A local immunological challenge in vivo (a graft-versus-host reaction) increased the activities of
hexokinase,
6-phosphofructokinase,
pyruvate kinase and
lactate dehydrogenase, confirming the importance of the glycolytic pathway in cell division. 2. The activities of the
ketone-body-utilizing
enzymes were lower than those of
hexokinase or
6-phosphofructokinase, unlike in muscle and brain, and were not affected by
starvation. It is suggested that the
ketone bodies will not provide a quantitatively important alternative fuel to
glucose in lymphocytes. 3. Of the
enzymes of the tricarboxylic acid cycle whose activities were measured, that of
oxoglutarate dehydrogenase was the lowest, yet its activity (about 4.0mumol/min per g dry wt. at 37 degrees C) was considerably greater than the flux through the cycle (0.5mumol/min per g calculated from oxygen consumption by incubated lymphocytes). The activity was decreased by
starvation, but that of
citrate synthase was increased by the local immunological challenge in vivo. It is suggested that the rate of the cycle would increase towards the capacity indicated by
oxoglutarate dehydrogenase in proliferating lymphocytes. 4.
Enzymes possibly involved in the pathway of
glutamine oxidation were measured in lymphocytes, which suggests that an
aminotransferase reaction(s) (probably
aspartate aminotransferase) is important in the conversion of
glutamate into oxoglutarate rather than
glutamate dehydrogenase, and that the maximum activity of
glutaminase is markedly in excess of the rate of
glutamine utilization by incubated lymphocytes. The activity of
glutaminase is increased by both
starvation and the local immunological challenge in vivo. This last finding suggests that metabolism of
glutamine via
glutaminase is important in proliferating lymphocytes.