1. The maximum catalytic activities of
fructose diphosphatase from flight muscles of bumble-bees (Bombus spp.) are at least 30-fold those reported for the
enzyme from other tissues. The maximum activity of
fructose diphosphatase in the flight muscle of any particular bee is similar to that of
phosphofructokinase in the same muscle, and the activity of
hexokinase is similar to or greater than the activity of
phosphofructokinase. There is no detectable activity of
glucose 6-phosphatase and only a very low activity of
glucose 6-phosphate
dehydrogenase in these muscles. The activities of both
fructose diphosphatase and
phosphofructokinase vary inversely with the
body weight of the bee, whereas that of
hexokinase is relatively constant. 2. There is no significant hydrolysis of
fructose 1-phosphate,
fructose 6-phosphate,
glucose 1,6-diphosphate and
glycerol 3-phosphate by extracts of bumble-bee flight muscle. 3.
Fructose 1,6-diphosphatase from bumble-bee flight muscle and from other muscles is inhibited by Mn(2+) and univalent
cations; the potency of inhibition by the latter varies in the order Li(+)>Na(+)>K(+). However, the
fructose diphosphatase from bumble-bee flight muscle is different from the
enzyme from other tissues in that it is not inhibited by
AMP. 4. The contents of
ATP,
hexose monophosphates,
fructose diphosphate and triose
phosphates in bumble-bee flight muscle showed no significant changes between rest and flight. 5. It is proposed that both
fructose diphosphatase and
phosphofructokinase are simultaneously active and catalyse a cycle between
fructose 6-phosphate and
fructose diphosphate in resting bumble-bee flight muscle. Such a cycle would produce continuous hydrolysis of
ATP, with the release of energy as heat, which would help to maintain the thoracic temperature during rest periods at a level adequate for flight.