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The level of an important carotenoid (beta-carotene) in the gut of Periplaneta americana depends on the content of the carotenoid in food: a carotenoid-fortified diet causes accumulation of beta-carotene up to 10 microg/g wet weight, while on a carotenoid-deficient diet the level of this substance is low (approximately 0.7 microg/g wet weight). In the eye, in contrast to the gut, a constant level of beta-carotene (1.3-1.4 microg/g wet weight) is found regardless of the diet. This phenomenon remained unchanged over three years of feeding of the cockroaches with the carotenoid-deficient diet, suggesting that P. americana produces carotenoids by de novo biosynthesis. This suggestion was confirmed in experiments using intraperitoneal injection of the exogenous carotenoid biosynthesis precursor [14C]mevalonic acid pyrophosphate followed by extraction of carotenoid and chromatographic purification of the labeled product. Injection of 3.4 nmoles [14C]mevalonic acid pyrophosphate transiently increased the beta-carotene content in eyes on days 2 and 4 after injection of the label. Purification of radiolabeled carotenoids from eye and gut by the transfer of carotenoids into a less polar solvent, alkaline hydrolysis (saponification), and chromatography on alumina and cellulose columns decreased the specific radioactivity to a constant level that cannot be further decreased by repeated chromatography. The elution profile of these purified preparations of beta-carotene after chromatography is characterized by coincidence of symmetric peaks of count and absorption. We suggest that to create the optimal carotenoid concentration in the eye, P. americana uses two biochemical mechanism: 1) it accumulates carotenoids in reserve in the gut when abundant supplies of carotenoids are available in the diet; 2) it synthesizes carotenoids de novo when its food is deficient in these compounds.