1. Activities of 3-oxo
acid CoA-
transferase and
carnitine palmitoyltransferase together with tri- and di-
acylglycerol lipase were present in red and heart muscles of the teleost fish. However,
d-3-hydroxybutyrate dehydrogenase activity was not detectable. These results suggest that the heart and red muscles of the teleosts should be able to utilize the fat fuels
triacylglycerol,
fatty acids or
acetoacetate, but not hydroxybutyrate. The muscles from the elasmobranchs differed in that
d-3-hydroxybutyrate dehydrogenase and 3-oxo
acid CoA-
transferase activities were present, but
carnitine palmitoyltransferase activity was not detectable. This suggests that
ketone bodies are the most important fat fuels in elasmobranchs. 2. The concentrations of
acetoacetate,
3-hydroxybutyrate,
glycerol, non-
esterified fatty acids and
triacylglycerols were measured in blood or plasma of several species of fish (teleosts and elasmobranchs) in the fed state. Teleosts have a 10-fold higher concentration of plasma non-
esterified fatty acids, but a lower blood concentration of
ketone bodies; both
acetoacetate and
3-hydroxybutyrate are present in blood of elasmobranchs, whereas
3-hydroxybutyrate is absent from that of the teleosts. 3. The effects of
starvation (up to 150 days) on the concentrations of blood metabolites were studied in a teleost (bass) and an elasmobranch (dogfish). In the bass there was a 60% decrease in
blood glucose after 100 and 150 days
starvation. In dogfish there was a large increase in the concentration of
ketone bodies, whereas in bass the concentration of
acetoacetate (the only
ketone body present) remained low (<0.04mm) throughout the period of
starvation. The concentration of plasma non-
esterified fatty acids increased in bass, but decreased in dogfish. These changes are consistent with the predictions based on the
enzyme-activity data. 4.
Starvation did not change the activities of
ketone-body-utilizing
enzymes or that of
phosphoenolpyruvate carboxykinase in heart and red skeletal muscles of both fish, but it decreased markedly the activity of
phosphoenolpyruvate carboxykinase in white skeletal muscle of both fish. However, in the liver of the dogfish,
starvation resulted in a twofold increase in the activities of
3-hydroxybutyrate dehydrogenase and
acetoacetyl-CoA thiolase, whereas in bass liver it decreased the activity of
acetoacetyl-CoA thiolase and increased that of 3-oxo
acid CoA-
transferase. The activity of
phosphoenolpyruvate carboxykinase was increased twofold in the liver of bass, but was unchanged in that of the dogfish. 5. The difference in changes in concentrations of blood metabolites and
enzyme activities in the two fish support the suggestion that, in
starvation,
ketone bodies, but not non-
esterified fatty acids, are an important fuel for muscle in elasmobranchs, whereas non-
esterified fatty acids, but not
ketone bodies, are an important fuel in teleosts. The results are discussed in relation to the evolution of a discrete
lipid-storing adipose tissue in teleosts and higher vertebrates.