Evidence for gluconeogenesis in the amphibian retina

Evidence for the existence of a gluconeogenic pathway was provided in the amphibian retina. It was found that [ 3H]glutamate was converted to [ 3H]glucose and the [ 3H]glucose derived from [ 3H]glutamate was incorporated into glycogen. The rate for this incorporation was found to be essentially the same in both light- and dark-adapted retinas: 0·147 vs. 0·142 nmol (mg protein × 2 hr) −1, respectively. However, the rate of incorporation was found to decline progressively with time. The rate for the incorporation of label derived from glutamate into glycogen was found to be considerably less than that for [ 3H]glucose: 10·2 nmol (mg protein × 2 hr) −1. The activity of a key gluconeogenic enzyme, fructose-1,6-bisphosphatase, also was demonstrated in retinal supernatants, approximately 1 nmol (mg × min) −1, and the activity of this enzyme was found to be inhibited both by adenosine monophosphate and by fructose-2,6-bisphosphate.