Steroid Amino acid uptake

Although cortisone interferes with protein synthesis in peripheral tissue, the hormone stimulates protein synthesis in liver. The increased rate of protein synthesis results from an effect of the hormone on the penetration of the amino acid through the cell membrane. The effects of the steroid hormones on the amino acid penetration were demonstrated with the aid of a-aminoisobutyric acid, a nonmetabolizable amino acid the intracellular penetration of which is regulated by mechanisms similar to those controlling glycine penetration. The penetration of a-aminoisobu-tyric acid is competitively inhibited by valine. Two hours after the injection of cortisol, the aminoisobu-tyric acid content of the liver is increased 70%. The increase in amino acid uptake may explain the increase of protein synthesis in liver observed early after cortisone injection. In that connection, it is of interest that an accelerated turnover of serum albumin has been reported early after the injection of cortisone. Since the concentrations in the levels of the blood protein are not changed, it seems that increased synthesis only compensates for increased catabolism. 

In the work of Loomeijer and work on similar lines carried out in the author s laboratory no evidence was found for the presence in important amounts of the common fatty esters, phospholipids, or steroids in purified preparations of elastin in these circumstances there seems to be little reason to classify elastin with the lipoproteins. The uptake of lipophilic dyes, which has been mentioned by several authors as indicating a lipoprotein character, can as well be explained by the presence of massive nonpolar radicals in the fluorescent prosthetic group, as described by Loomeijer, or to interactions with lipophilic sections of the peptide chain due to the large concentrations of nonpolar amino acids such as valine, the leucines, and phenylalanine. 

For example, glucocorticoids stimulate gluconeogenesis, lipolysis, and an increased uptake of amino acids by the liver.) Cortisol also possesses a small amount of mineralocorticoid activity. (Mineralocorticoids regulate Na+ and K+ metabolism. For example, aldosterone, the most important mineralocorticoid in humans, induces the reabsorption of Na+ from urine. It also promotes the secretion of K+ and H+ into urine.) For steroids to have either glucocorticoid or mineralocorticoid activity, they must possess a hydroxy group at C-ll. 

It was suggested above that steroid hormones act by stimulating bioenergetic pathways, and that as a result of this stimulation, the biosynthesis of phospholipids, proteins, and nucleic acids is activated. After estrogen administration there is a rapid uptake of precursors P in phospholipids, glycine or other amino acids into proteins, and serine and formate into the purine and pyrimidines of the nucleic acid. 

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