No abstract

The basis for the variation in fatty acid composition in different ovine adipose tissue depots was investigated. The proportion of stearic (C18:0) and oleic (C18:1) acids vary in a site-specific fashion; abdominal depots (omental and perirenal) contain relatively more C18:0 than C18:1, and carcass depots, especially sternum, have a markedly higher proportion of C18:1. Additionally, expression of a number of lipogenic enzyme genes (stearoyl-CoA desaturase [SCD], acetyl-CoA carboxylase-alpha [ACC-alpha], lipoprotein lipase [LPL]) and the cytoskeletal protein gene alpha-tubulin vary among depots, although the pattern of variation differs for each mRNA. When these expression data were related to the mean cell volume of adipocytes pooled from all depots, a significant pattern emerged: expression of the ACC-alpha, LPL, and alpha-tubulin genes was highly correlated with the size of adipocytes. In contrast, when the expression of SCD mRNA was assessed as a function of mean cell volume, two populations of adipocytes emerged: no significant correlation was found between the expression of SCD mRNA per adipocyte and mean cell volume for the abdominal depots, although a highly significant correlation was observed between SCD gene expression and mean cell volume for the carcass and epicardial depots. Similarly, a highly significant correlation was found for the amount of C18:1 per adipocyte and the abundance of SCD mRNA per adipocyte for the carcass and epicardial depots, whereas no significant correlation was observed for these traits for the omental and perirenal depots. Thus, the SCD gene seems to be regulated in a depot-specific fashion and in a manner distinct from that of the ACC and LPL genes.

Sheep adipose tissue explants were maintained in culture for 24 h in the presence of insulin, dexamethasone, or insulin and dexamethasone, and stearoyl-CoA desaturase (SCD) messenger RNA (mRNA) levels and fatty acid synthesis were measured. Insulin increased SCD mRNA levels (P = 0.008) and synthesis of both saturated (P = 0.07) and unsaturated (P < 0.001) fatty acids but had the greatest effect on unsaturated fatty acid synthesis, resulting in the overall production of a greater (P < 0.001) proportion of monounsaturated fat. Dexamethasone, alone, had the opposite effect but actually potentiated the effect of insulin in stimulating SCD expression and both saturated and monounsaturated fatty acid synthesis, without affecting the relative proportions of each. Across adipose tissue depots, the effect of hormones was similar, although the increase in SCD mRNA levels (P = 0.008) and monounsaturated fatty acid synthesis (P < 0.001) was greater in subcutaneous adipose tissue than in the internal (omental and perirenal) depots. These data clearly show that, in ovine adipose tissue, changes in SCD gene expression in response to insulin and dexamethasone are associated with changes in monounsaturated fatty acid synthesis and suggest that it may be possible to develop strategies to manipulate sheep tissues to produce a less-saturated fatty acid profile.

Considerable evidence exists to suggest that not all long chain saturated fatty acids are metabolized in an identical manner, for example, dietary stearic acid does not increase plasma cholesterol to the same extent as palmitic acid [I] Several reasons have been suggested for this difference, including effects on intestinal absorption [2], rate of desaturation [3] and rate of incorporation into cellular lipids [4] We have previously reported reduced plasma VLDL cholesterol concentrations in hamsters fed tristearin compared to tripalmitin [5]In the present study this was investigated further by considering the desaturation and esterification of these fatty acids, in comparison to oleic acid, by monolayer cultures of hamster hepatocytesHamster hepatocytes were prepared and maintained in monolayer culture essentially as previously described for rat hepatocytes [6] After overnight incubation, cells were transferred to serum free medium containing 2gA bovine serum albumin (BSA) and 300pM fatty acid bound to BSA (61 Cells were incubated for up to 8h, after which lipids were extracted from both cells and medium and individual lipid classes were separated by thin layer chromatography Table I: Removal of fatty acid fmm the medium and estenfication into differrent lipid fractions in cultured hepatocytes medium and secreted triacylglycerol and nMoles fatty a c i d h h g cell protein for incorporation into cellular triacylglycerol and phospholipid Removal of fatty acids from the medium and incorporation into cellular lipids was measured over 4h while secretion was measured over 8h Table I shows that palmitate was removed from the medium at a faster rate than oleate or stearate The rate of incorporation of fatty acids into cellular triacylglycerol and phospholipid was linear for up to 4h and decreased in the order, palmitate>oleate>stearate The proportion of stearate which was incorporated into phospholipid relative to triacylglycerol was similar for oleate (0 52) and palmitate (0 53), but considerably higher for stearate (0 69) Incorporation of fatty acid into secreted triacylglycerol was linear for up to 8h The rate of secretion was similar for oleate and palmitate but considerably lower for stearate. The relative proportion of the total triacylglycerol which was secreted at the 4h time point was greater for oleate (21. I I"/) than for palmitate and stearate ( 1 5.28 and 17.75%, respectively). incorporation of each of the radiolabelled fatty acids into secreted phospholipid appeared to saturate at 30min. without any further increase with time.In a further experiment the extent of desaturation of palmitate and stearate was examined.Hepatocytes were incubated with fatty acids as indicated above. After 4h cellular lipids were extracted and separated, and the phospholipid and triacylglycerol fractions were then saponified. The fatty acids released were separated by silver phase thin layer chromatography and radioactivity associated with saturated and monounsaturated bands determined. Table 2 shows that more stearate than palmit...

Lamb consumption fell from 11.1 to 7.5 kg / year / person between 1960 and 1990 and in recent years has fallen even further (5.9kg / year / person in 1994, source MLC). There are a number of possible reasons for this, but the high saturated fat content of lamb is likely to be a major contributer. Health considerations have led to repeated calls to reduce saturated fat intake (Dept of Health, 1994). Lamb has particularly suffered because it has a higher content of the saturated fatty acid stearic acid (C18:0) than other red meats. This has a high melting point and gives cold lamb its unpopular texture. The majority of lipid in lamb comes from de-novo synthesis of the saturated fatty acid palmitate (C16:0) from acetate in the adipose tissue by the enzymes acetyl co-A carboxylase and fatty acid synthetase (FAS). Palmitate can be elongated to stearate (C18:0) and this can be desaturated to the potentially healthier monounsaturated fatty acid oleate (C18:l), by the enzyme stearoyl co-A desaturase (SCD). The aim of this work has been to examine the fatty acid profile of lamb during growth and the enzymes involved in fat deposition.