Lipoprotein lipase preparations were obtained by aqueous extraction of various tissues from sheep and rats. Preparations having high activity towards serum-activated triolein emulsions were obtained from actively growing sheep, provided that the tissue was maintained at 37�C throughout the extraction procedure. Activity of lipoprotein lipase from sheep adipose tissue, like that from the rat, was dependent upon the presence of serum in the reaction mixture, and was optimal at pH 8-9. Inhibition of sheep adipose tissue preparations by protamine sulfate (1 mg/ml) and O' 6 M NaCl was similar to that found for rat adipose tissue preparations. The affinity of the lipoprotein lipase preparation for the activated substrate from sheep and rat adipose tissue was, however, markedly different; sheep preparations being activated at much lower substrate concentrations (Km O' 43 mM) than rat preparations (Km > 5 mM). These findings, confirmed with acetone--ether preparations of lipoprotein lipase, indicate that the enzyme from sheep adipose tissue has a greater potential to remove triacylglycerol from the plasma. This could result in a greater deposition of fat in the tissue.

Lipoprotein lipase and hepatic lipase have been shown to be present in the post-heparin plasma of sheep. Intravenous injection of heparin into sheep produced a rapid increase in the free fatty acid concentration and lipolytic enzyme activity of the plasma, both peaking within 5-15 min and then falling to pre-heparin levels within 30-60 min. Lipolytic activity was not detected in plasma before heparin treatment. Two distinct lipolytic activities were separated from the plasma by chromatography on heparin-Sepharose 6B. Lipoprotein lipase was identified on the basis that the lipolytic activity was dependent upon the addition of plasma, inhibited by 1M NaCI, and inhibited by a specific antiserum against lipoprotein lipase. The second lipolytic activity of plasma was identified as hepatic lipase, as it was not dependent upon plasma for activity, nor was it inhibited by 1M NaCI or antiserum against lipoprotein lipase. Its properties were identical to the lipase extracted from the liver of sheep. Lipoprotein-lipase activity, but not hepatic-lipase activity, was dependent upon the nutritional state of the sheep at the time of heparin injection. However, hepatic lipase comprised a significant proportion of the total lipolytic activity.

The effects of species and plane of nutrition on serum activation of sheep adipose tissue lipoprotein lipase were studied over a range of substrate (triolein) concentrations. Serum, either from two species or from the same species on a different plane of nutrition, had differing effects on adipose tissue lipoprotein lipase activity. Serum from fed sheep was more effective than serum from fed rats in activating sheep adipose tissue lipoprotein lipase at low substrate concentrations. Serum taken from sheep on a restricted plane of nutrition, stimulated adipose tissue lipoprotein lipase activity at physiological substrate concentrations. The increased activity promoted by the factor(s) present in serum would ensure that those tissues (e.g. cardiac and skeletal muscle) which continue to synthesize lipoprotein lipase during fasting or nutritional restriction, are able to assimilate the relatively low amounts of circulating triacylglycerol.

Two distinct activator proteins for lipoprotein lipase (LPL) have been isolated in approximately equal amounts from ovine plasma. These low molecular weight proteins were readily separated from each other on the basis of size and charge. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis indicated proteins of Mr about 8000 and 5000, with pI in urea-containing gels of about 5·1 and 4·8 respectively. Each of the ovine activator proteins was as effective as human apolipoprotein C-II (apo C-II) in activating LPL, with I JLg/ml giving near to maximum activation, and in lowering the apparent Km of LPL for triolein substrate. As the ratio of activator to triolein increased from 0·16 to 5· 2 (JLg/mg) the apparent Km fell from about O' 5 to 0 ·18 mM. Whereas human apo C-II and the two ovine activators were equally effective in stimulating the hydrolysis of triolein, differences were observed between the human and ovine activators when p-nitrophenylbutyrate was used as substrate. Unlike human apo C-II, which produced significant inhibition of p-nitrophenylbutyrate hydrolysis, the ovine activators were without effect. This suggests that the interaction between the ovine activators and LPL is different from that of human apo C-II.

The activity of the enzyme aspartate carbamoyltransferase (ACT; EC 2.1.3.2) was studied in ovine tissues. Hepatic, ileal and duodenal tissues showed higher levels of activity (6-19 units) than skeletal muscle, cardiac muscle, lung, spleen, kidney and rumen wall (2-7 units). Low growth impetus skeletal muscles had higher activity (2�9 units) than high growth impetus muscles (1�8 units) taken from immature sheep but such differences were not present in mature sheep.