D M McGuire

Role of sterol carrier protein in cholesterol metabolism

This report summarizes our recent studies on the protein known as sterol carrier protein (SCP) or fatty acid binding protein (FABP). SCP is a highly abundant, ubiquitous protein with multifunctional roles in the regulation of lipid metabolism and transport. SCP in vitro activates membrane-bound enzymes catalyzing cholesterol synthesis and metabolism, as well as those catalyzing long chain fatty acid metabolism. SCP also binds cholesterol and fatty acids with high affinity and rapidly penetrates cholesterol containing model membranes. Studies in vivo showed SCP undergoes a remarkable diurnal cycle in level and synthesis, induced by hormones and regulated in liver by translational events. SCP rapidly responds in vivo to physiological events and manipulations affecting lipid metabolism by changes in level. Thus SCP appears to be an important regulator of lipid metabolism. Preliminary evidence is presented that SCP is secreted by liver and intestine into blood and then taken up by tissues requiring SCP but incapable of adequate SCP synthesis.

On the mechanism of the alterations of rat kidney transamidinase activities by diet and hormones.

The first reaction in the biosynthesis of creatine is the transfer of the amidine group of arginine to the amino group of glycine to form ornithine and guanidinoacetic acid. This reaction is catalyzed by the enzyme L-arginine: glycine amidinotransferase, EC 2.1.4.1, commonly called transamidinase. Transamidinase was discovered by Borsook and Dubnoff in 19411 and the only tissues from the rat that have significant transamidinase activities are kidney and pancreas 1–5. The second reaction in the biosynthesis of creatine is the methylation of guanidinoacetic acid in the liver by S-adenosylmethionine catalyzed by the enzyme S-adenosylmethionine: guanidinoacetate N-methyltransferase, EC 2.1.1.2. Hormonal induced alterations in this enzyme activity have not been found6. Livers from rats fed complete diets supplemented with creatine had similar guanidinoacetate methyltransferase activities as from rats fed the complete diet without creatine7. Rat kidney transamidinase activities have been found to be altered greatly in a variety of dietary and hormonal states. Thus transamidination is thought to be the control step in creatine biosynthesis. Since ornithine is a product of this enzyme reaction, an interrelationship may exist between the control of creatine and urea synthesis.

The Existence of Multiple Forms of Rat Kidney Transamidinase

Two forms of rat kidney transamidinase, called α and β, have been purified to homogeneity1. No differences were found in the properties of these forms other than their separation from each other by chromatography on DEAE cellulose. Polyclonal antibodies made to the α-form of the enzyme reacted with the β-form of the enzyme and precipitated all of the enzyme activity from a rat kidney homogenate. The low transamidinase activities in kidneys from rats fed creatine-supplemented diets correlated well with the relative amounts of transamidinase protein as determined by immunotitration with the polyclonal antibodies2. To farther examine α and β transamidinase regulation, monoclonal antibodies to rat kidney transamidinase were made3 and used to determine the relative amounts of transamidinase protein in kidneys from normal and creatine-fed rats. The results are presented in this report. Also, the techniques of isoelectric focusing and Western blotting with either the monoclonal or the polyclonal antibodies were used to further characterize rat kidney transamidinase.