Contribution of the pentose cycle to glucose metabolism by insects

Abstract

• 1.

  1. The rates of in vivo conversion of variously labelled glucose, acetate and gluconate to ¹⁴CO₂ by the cockroach (Periplaneta americana) milkweed bug (Oncopeltus fasciatus) and grasshopper (Melanoplus bivittatus) suggest the occurrence of an active pentose cycle and glycolytic-critic acid cycle in these insects.

• 2.

  2. The activitiy of the pentose cycle in the male and female cockroach was quantitated by two independent methods; one based on ¹⁴CO₂ yields from the variously labelled glucose and the other based on the degree of randomization of ¹⁴C between C-1 and C-3 of glucose residues of glycogen when the pentose cycle was challenged with glucose-2-¹⁴C. The results were consistent in that in the male cockroach about 21 per cent and in the female about 3 per cent of the glucose was metabolized by the pentose cycle.

• 3.

  3. About 50 per cent of the glucose in the male cockroach was metabolized by nontriose pathways.

• 4.

  4. The contribution of the pentose cycle was about 18 per cent in the milkweed bug and 38 per cent with the grasshopper.

• 5.

  5. The labelling pattern of the glucose carbon atoms suggests (a) complete equilibrium between glucose-6-P and fructose-6-P in the male but only patial equilibration in the female and (b) that attempts to quantitate the pentose cycle activity through application of equations derived for mammalian tissue suffer interference from the transketolase reaction and from reversal of non-oxidative portions of the pentose cycle.