Activities of 3‐Hydroxybutyrate Dehydrogenase, 3‐Oxoacid CoA‐Transferase and Acetoacetyl‐CoA Thiolase in Relation to Ketone‐Body Utilisation in Muscles from Vertebrates and Invertebrates

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Abstract
The activities of 3-hydroxybutyrate dehydrogenase were non-detectable in muscles of invertebrates and marine teleost fish; activities were found in muscles of amphibia, reptiles and mammals and also in an elasmobranch fish. Muscles were classified into 3 groups according to the activities of 3-oxoacid CoA-transferase: muscles with very low activities (< 0.01 .mu.mol .times. min-1 .times. g-1), which obtain energy for contraction from anaerobic glycolysis; muscles with low activities (> 0.01 < 5 .mu.mol .times. min-1 .times. g-1), which include insect flight muscles, muscles of other invertebrates and skeletal muscles of higher vertebrates; and muscles with high activities of 3-oxoacid CoA-transferase (> 5 .mu.mol .times. min-1 .times. g-1), which are characterized by continuous mechanical activity for long periods of time, e.g. heart, diaphragm, postural and some smooth muscles of mammals. Ketone bodies may be important fuels for muscles in the very low and low activity groups during starvation when the muscle is at rest. The muscles in the high activity group may use ketone bodies when they are available in the blood to provide energy for mechanical activity. Since these muscles provide a continuous vital physiological function, they must always be provided with a fuel for respiration and, in a similar manner to brain, they may utilize either glucose or ketone bodies.