Lipid metabolism in helminth parasites. II. The major origins of the lipids of Hymenolepis diminuta (Cestoda).

H. diminuta readily absorbed and incorporated label from 14C-starch and l-`4C-palmitate administered in a synthetic diet to infected rats. In vitro, it incorporated label from glucose, palmitate, and l-lC-acetate. Carbon from starch or glucose appeared chiefly in glycogen or excretion products, although significant amounts were converted to water-soluble components of lipids, such as glycerol and inositol. Specific activities were higher in phospholipids, and particularly in phosphoinositides, than in glycerides. A series of experiments designed to test the incorporation of dietary carbohydrate carbon into higher fatty acids was uniformly negative. Both palmitate and acetate-carbon, on the other hand, appeared in glyceride and phospholipid fatty acids. Present evidence indicates that H. diminuta has only a limited capacity for fatty acid biosynthesis, possibly by chain lengthening rather than de novo synthesis, and that most of its abundant supply of tissue acids are dietary. Although the host pellet diet contained a fatty acid mixture highly unlike that of the tapeworm, the rat intestinal content fatty acids were similar. This profound modification may have been due to the fatty acids of rat bile lipids, chiefly lecithin. It appears that the parasite habitat is fairly uniform with respect to fatty acids and that the parasite fatty acids are similar to those of its environment, although some degree of control is evident. The medium in which H. diminuta was cultured in vitro contained no lipids except those firmly bound to several blood protein fractions. Fatty acids in these lipids were similar to those of H. diminuta and were present in amounts sufficient to satisfy the tapeworm's needs. Ginger and Fairbaim (1966) presented a detailed study of the lipids of Hymenolepis diminuta, in which they confirmed the observation (Harrington, 1965) that approximately 80% of the fatty acids of this adult cestode are unsaturated, despite the fact that its habitat, the rat intestine, is deficient in oxygen. As oxygen is known to be required for the biosynthesis of unsaturated fatty acids in other animals (Goldfine and Bloch, 1963), it was pointed out that the parasite's fatty acids might originate (1) in the host diet, or in substrates provided by the host, (2) in biosyntheses making efficient use of oxygen at low partial pressures, or (3) in biosyntheses making use of undescribed mechanisms. Clearly, these possibilities are not mutually exclusive, but may be operative in varying degrees, alone or in combination, depending upon external conditions. In other metazoans, for example, dietary fatty acids are readily absorbed and incorporated into lipids, but are also biosynthesized from carbohydrate and other sources of suitable twoor three-carbon precursors. Biosynthetic capability, by alteration of carbon chain length and Received for publication 20 July 1966. * With financial assistance from Grants No. 5 TI AI 226 and AI 04953, NIH. t Permanent address: Department of Parasitology, National Institute for Medical Research, The Ridgeway, Mill Hill, London, N.W. 7. degree of unsaturation, also enables the organism to maintain control, within limits prescribed by the nature and quality of dietary acids, of its fatty acid composition. As a first step towards understanding the origins of parasite lipids, a number of in vivo and in vitro experiments have been performed with H. diminuta in which the absorption, assimilation, and interconversions of substances such as starch, glucose, glycogen, acetate, and palmitic acid have been examined.