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

1 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 three groups according to the activities of 3‐oxoacid CoA‐transferase: muscles with very low activities (> 0.01 μmol × min −1 × g −1 ) which obtain energy for contraction from anaerobic glycolysis; muscles with low activities (< 0.01 >5 μmol × min −1 × g −1 ) which include insect flight muscles, muscles of other invertebrates and skeletal muscles of higher vertebrates; muscles with high activities of 3‐oxoacid CoA‐transferase (< 5 μmol × min −1 × g −1 ) which are characterised by continuous mechanical activity for long periods of time, e.g. heart, diaphragm, postural and some smooth muscles of mammals. 2 It is suggested that 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 utilise either glucose or ketone bodies.