PRIMITIVE ACTINOPTERIGIAN FISHES CAN SYNTHESIZE ASCORBIC-ACID

Amphibians and reptiles evolved with the capacity to synthesize ascorb ic acid. Some higher vertebrates, like bats, guinea pigs, primates, an d humans have lost the microsomal enzyme gulonolactone oxidase, and in cases of ascorbic acid deficiency suffer from symptoms of scurvy. The question of whether the capacity to synthesize ascorbate is also pres ent in lower vertebrates could throw light on the evolution of this pa thway. In order to find out whether ascorbic acid synthesis took place in two primitive Actinopterigian fish, the paddlefish (Polyodon spath ula) and the white sturgeon (Acipenser transmontanus) were fed with a scorbutogenic diet or diet(s) supplemented with a graded level of asco rbic acid. We found no growth depression nor external symptoms of scur vy, which would be pronounced in modern bony fishes (Teleostei) under similar conditions. The tissue level of ascorbate in both these primit ive species indicated that vitamin C in intestine and liver is not dep leted when fed a scorbutogenic diet. Gulonolactone oxidase activity wa s found in the kidneys of the Actinopterigian fishes. Thus, I question the accepted evolutionary pathway for ascorbic acid biosynthesis in l ower vertebrates and suggest that the modern bony fishes, Teleostei, l ost their ability to express the gulonolactone oxidase genes after the y had separated during the Silurian from their common ancestor with th e coelacanths (Latimeria) and Dipnoi.