Samuel Gurin

Structure of Lipoproteins: Covalently Bound Fatty Acids

Plasma lipoproteins, extensively extracted with organic solvents, contain small quantities of firmly bound long-chain fatty acids. Upon enzymatic digestion of the protein and partial purification of the resulting peptides, fatty acids are found associated with a peptide fraction which contains organic phosphate. The nature of the linkage between the fatty acid and peptide is unknown.

The incorporation of radioactive acetate into biliary cholesterol and cholic acid

Abstract 1-14C sodium acetate and 2-14C sodium acetate were administered through the portal vein to animals whose common bile duet had been cannulated. Daily samples of the bile were collected after treatment with the labelled acetate. Biliary cholesterol and cholic acid were isolated from each sample and the specific activities of each were measured. The specific activity of the cholic acid was found to be greater than the specific activity of cholesterol in a majority of the samples measured. Cholic acid preparations obtained from 1-14C sodium acetate and 2-14C sodium acetate were decarboxylated and the specific activities of the carboxyl carbon atoms determined. The specific activities of the carboxyl were consistent with the assumption that cholic acid is derived from cholesterol by cleavage of the three terminal carbons of the side-chain.

Carbohydrates of the Gonadotropic Hormones

Summary A qualitative as well as a quantitative study has been made of the hexose present in the gonadotropic hormones of the pituitary gland, pregnant mare serum and human pregnancy urine. The hormones obtained from the pituitary gland contain mannose and hexosamine in equimolar proportions. The gonadotropins of pregnant mare serum and human pregnancy urine appear to contain galactose rather than mannose. In these preparations the molar ratio of hexose to hexosamine is 2:1.

Biosynthesis of Radioactive Allantoin.

Summary 1. A convenient method for the isolation of allantoin from the urine of rats has been developed. 2. The carbon of sodium bicarbonate is not incorporated in the urinary allantoin of rats. This result is in accord with the fact that bicarbonate is incorporated only into position 6 of purines. 3. The carboxyl carbon of acetate is not involved in the biosynthesis of allantoin in the rat. 4. The carboxyl carbon of glycine is incorporated into position 4 of allantoin and the methylene carbon into position 5.

Isotopic Tracers in the Study of Carbohydrate Metabolism

Publisher Summary This chapter discusses isotopic tracers in the study of carbohydrate metabolism. Balance studies carried out both in vivo and in vitro upon plants as well as animals have enabled the biologist and biochemist to amass a great deal of information concerning metabolic processes. The ingestion of acetate by an animal produces no apparent effect upon the biological synthesis of carbohydrate. Administration to the animals of acetate labeled with carbon (C) 12 results in the formation of isotopic glycogen even though there has been no net increase in glycogen synthesis. The fasted, phlorizinized rat excretes an increased amount of urinary D-glucose when the amino acid alanine is administered; nevertheless, the extra D-glucose excreted contains very few of the carbon atoms originally furnished by the administered D,L-alanine.

Normal Lipid Metabolism

Very striking advances have been made recently in our understanding of lipid metabolism. Much of this has come about because biochemists have learned how to prepare cell-free preparations without producing too extensive destruction of intracellular enzyme systems. The tremendous advantages of this type of technic are obvious. Once a cell-free preparation is available, the various particulate fractions of the cell can be separately studied for enzymatic activity. If, in addition to this, it becomes possible to extract from these particles the water-soluble enzymes capable of performing a series of metabolic reactions, then the way is open for a final attack. Every energy of the chemist is devoted to obtaining aqueous solutions of such enzyme mixtures since this permits him to fractionate and separate individual enzymes. Once this has been achieved, it becomes possible to study single reactions of an otherwise complex series of metabolic reactions. With a single enzyme, the reaction product accumulates; in a mixture of enzymes, the reaction product is metabolized further and does not accumulate. It is obvious that studies with the whole animal or with a whole organ can provide answers concerning starting material and end-products. Almost never can such studies yield direct evidence of the nature of the intervening reactions. It is a source of great satisfaction that both the oxidation of fatty acids as well as their synthesis have now been accomplished in aqueous particle-free enzyme solutions. Such studies have demonstrated conclusively that the initial oxidative attack upon fats involves the formation of activated fatty acids. Such activated acids have now been shown to be combined through their acid groups with coenzyme A (a pantothenic acid derivative containing a sulfhydryl group). The resulting activated product is a thiol ester (figure i ) which is now capable of undergoing the classical beta oxidation process. The important point to remember is that, although this involves the usual beta-hydroxy and beta-keto acids as intermediates, the coenzyme A group still remains firmly