CARBOHYDRATE METABOLISM IN THE EGGS OF THE SILKWORM, BOMBYX MORI. I. ABSENCE OF PHOSPHOFRUCTOKINASE IN THE DIAPAUSING EGG

In the diapausing eggs of the silkworm, Bombyx mori , glycogen is rapidly converted to sorbitol and glycerol, and this conversion is reversed at termination of the diapause (C hino , 1958). To elucidate the pathway leading to this polyol formation and its regulatory mechanisms, enzymes concerning carbohydrate metabolism were surveyed in diapausing as well as in developing eggs of the silkworm.
Most of the enzyme activities concerning citric acid cycle are low at the beginning of the embryogenesis and during diapause, but increase at the later stages of the development. Making an exception, reduction rate of malate and fumarate was rather high from the onset of the embryonic development. Several glycolytic enzymes were also studied. Most remarkable fact is that phosphofructokinase activity could not be demonstrated in the diapausing and also in the early stages of the developing eggs. Other enzymes, viz. α-glycerophosphate dehydrogenase, aldolase, glyceraldehyde-3-phosphate dehydrogenase were detected from the beginning of the embryogenesis.
Absence of phosphofructokinase, together with the high activity in glucose-6-phosphate dehydrogenase, suggests that predominant pathway in carbohydrate metabolism in the early stages of embryogenesis and in the diapause period is by way of pentose phosphate pathway. This supposition is confirmed by the experiments using labeled glucose. Incorporation of the label into glycerol of the diapausing eggs was three to four fold when G-6-¹⁴C was injected into pupae as compared with the case of G-1-¹⁴C injection. The above experiments provide evidence supporting the theory that glycogen is converted into sorbitol and glycerol mostly by way of the pentose phosphate pathway in the diapausing eggs.