Abstract
1. 1. Factors affecting 14C-bicarbonate incorporation into urea in active and fasting slugs, Limax flavus, were investigated. 2. 2. Since a high percentage of the foodstuffs catabolized during fasting was protein, and since fasting slugs synthesized urea faster than active animals, increased substrate availability from protein breakdown was assumed to account for the greater incorporation rates. 3. 3. Enzyme activities of glutamine synthetase and the urea cycle enzymes were not altered with fasting. 4. 4. Injections of urea cycle intermediates (ammonia, ornithine, aspartic acid and arginine) all accelerated urea production, while no response was detected for glutamine and glutamic acid. 5. 5. Apparent K m s of the enzymes studies are within the physiological range of metabolite concentrations and therefore allow for sensitive response to changing substrate concentrations. 6. 6. That the slug, L. flavus, produced ample quantities of urea during both active feeding and starvation and does not have an active urease suggests that slugs differ considerably from many other gastropods in their nitrogen catabolism.
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