THE FATE OF SOME C-14-LABELED DIETARY LIPIDS IN THE TIGER PRAWN PENAEUS-ESCULENTUS

Penaeus esculentus Haswell were collected from Moreton Bay, Queensland, Australia, by trawling during 1987-1988. After standardisation for nutritional state and moult stage (C to D0), prawns were fed one of three C-14 labelled lipids: a non-essential fatty acid (FA: palmitic acid, 16:0); an essential polyunsaturated fatty acid (PUFA: eicosapentaenoic acid, 20:5n-3); and the essential sterol, cholesterol. The diet was not deficient in any of these lipids. Experiments were run for 24 and 72 h, the rate of production and radioactivity of CO2 was determined, and at the end of each experiment lipid classes of the neutral (NL) and polar lipid (PL) fractions of the digestive gland, abdominal muscle and integument were separated and the C-14 counted. Approximately 34% of the C-14 16:0 and C-14 20:5n-3 was oxidised to CO2 at similar rates up to 72 h, both reaching a peak at 12 h; cholesterol was not oxidised to CO2. The distribution in the tissues of C-14 from all three labels was similar: digestive gland > muscle > remainder > blood > gills > proventriculus > hindgut. This was not due to mass of the tissue or its lipid content. Most of the label from the FA was in PL (digestive gland > 50%, muscle and integument > 80%). In the NL, most of the label was in free FA; in the PL classes, C-14 was predominant in phosphatidyl-choline (C), especially in the muscle and integument. The data indicate that the digestive gland is a major site of lipid synthesis, as well as assimilation and storage. The distribution of the C-14 20:5n-3 label differed appreciably from that of C-14 16:0 only in the PL fraction, where it was more evenly distributed. It was concluded that, when in excess, the fate of this essential PUFA is similar to that of non-essential FA. Labelled cholesterol was distributed readily through the tissues, but appeared to be mostly retained as such.