Biochemical SocietyTransacttons ( 1 992)20 Lipoprotein lipase activity developing chick embryo
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BRIAN K. SPEAKE, RAYMOND C. NOBLE and RUTH McCARTNEY Drpartment of Biochemical Sciences, Scottish Agricultural College, Auchincruive Ayr, Scotland, KA6 5HW, U.K. The lipids of the yolk provide the chick embryo with over 90% of its energy needs as well as with structural components and essential fatty acids required for tissue growth and development. The transfer of lipid from the yolk to the embryo is mediated by an extraembryonic structure, the highly vascularised yolk sac membrane. Yolk lipid is taken up into the cells of the yolk sac membrane, apparently by phagocytosis, and is subsequently released as plasma lipoprotein particles into adjacent blood capillaries for delivery to the embA-yoniccirculation. This process is especially intensive during the last 8 days of the 21 day developmental period [l]. Although chick embryo development is characterised by distinctive and often dramatic changes in tissue-specific lipid composition {l], there is little information regarding the mechanisms of uptake and utilisation of yolk-derived lipid by the embryonic tissues. In adult mammals and l'rds, the initial stages in the processing of triacylglycerol-rich plasma 1 ipoproteins are catalysed by lipoprotein lipase present at the luminal surface of the blood Fatty acids capillaries of specific tissues [2]. released by the action of this enzyme are thus made available for utilisation by these particular tissues. In the light of a previous report [3] suggesting a role for lipoprotein lipase in chick embryo lipid metabolism we have investigated several tissues of the embryo for the presence of this enzyme at various stages of development. A preliminary study, using embryos at the 14th day of development, indicated that lipoprotein lipase was present at a high specific activity in adipose tissue and heart. The enzyme was also detectable in skeletal muscle, but was absent from brain and liver. In subsequent investigations the activity of the enzyme in adipose tissue and heart was monitored throughout the developmental period. Tissue from several embryos (5-20, depending on the developmental stage) was pooled, homogenised in 10 vols. of 5mM sodium barbital, pH7.5, containing 2VA (v/v) glycerol, 0.1% (v/v) Triton X-100 and 50mM NaC1, and lipoprotein lipase activity was determined [4]. A major increase in the activity (expressed as unitddepot) of lipoprotein lipase in adipose tissue occurs between days 12 and 16 of development (Table 1). concomitant with the beginning of the period of lipid uptake from the yolk and also with a period of intensive deposition of triacylglycerol into the A further increase in enzyme level adipocytes. occurred after hatching, presumably induced by food intake [2]. The biphasic nature of the developmental changes in lipoprotein lipase levels was also apparent when the enzyme activity was expressed as units/g tissue, although the activity thus expressed decreased between days 16 and 19 due to a rapid increase in the tissue weight and lipid content over this period. The activity of lipoprotein lipase in the heart, expressed as units/g tissue increased from approx. 5 units at day 7 to approx 30 units at day 14. Thereafter, increases in the enzyme's activity occurred largely in parallel with the growth of the heart, so that no major changes in activity/g tissue took place. Analysis of the fatty acid composition of +,he triacylglycerol fraction of both adipose tissue and heart, using gas-liquid chromatography, revealed the presence of relatively high levels (up to 8% by weight of total fatty acids) of docosahexaenoic acid
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Table 1. Lipoprotein lipase activity of chick embryo adipose tissue Subcutaneous adipose tissue from the upper thigh area and along the sides of the abdomen and thorax was used as the source of the enzyme. The combined tissue from both sides of a single embryo is referred to as a depot. Units of enzyme activity refer to umoles fatty acid released/h. Each value represents the mean (2S.D.) of 3 separate experiments. Lipoprotein lipase activity Days of Development
12 16 19 5 days post-hatch
Units/g tissue wet wt.
32.4L16.9 73.7i 1.8 34.02 9.0 69.5i10.9
Unitddepot 1.120.3 22.2L0.8 21.2i4.2 65.7L7.1
(22:6,n-3) between days 12 and 16 of development. Since yolk triacylglycerol contains negligible proportions of this fatty acid it would appear that certain modifications in lipid composition occur during the conversion of yolk lipid droplets to plasma lipoproteins by the yolk sac membrane [l]. In conclusion, the results suggest a major role for lipoprotein lipase in the tissue-specific utilisation of yolk-derived lipid by the chick embryo. SAC received financial support from the Office Agriculture and Fisheries Department.
Scottish
1. Noble, R.C. & Cocchi, M. (1991) Prog. Lipid Res. 29, 107-140. 2. Speake, B.K., Parkin, S.M. & Robinson, D.S. (1985) Biochem. SOC. Trans. 13, 29-31. 3. Bengtsson, G . , Hernell, 0. & Olivecrona, T. (1977) Int. J. Biochem. 8, 587-539. 4. Parkin, S.M., Speake, B.K. & Robinson, D.S. (1982) Biochem. J. 207. 485-495.