Protein metabolism, human

Human exposure to low levels of phenol is widespread because it is contained in many consumer products including mouthwashes, gargles, tooth drops, throat lozenges, and ointments (Douglas 1972 EPA 1980). Phenol is a normal product of protein metabolism, and it is also a metabolite of benzene. In persons not exposed to phenol or benzene, the total phenol concentration in the urine generally does not exceed 20 mg/L and is usually <10 mg/L (ACGIH 1991). 

CYP2C8 has recently been established at the principal enzyme responsible for the 6a-hydroxylation of taxol, the principal route of metabolism for this anticancer drug (Rahman et al., 1994). For this study, a combined approach of correlation analysis with levels of immunoquantitated CYP2C8 protein in human liver microsomes, inhibition analyses in human liver microsomes, and vv-expressed proteins established that CYP2C8 was the principal enzyme for taxol 6a-hydroxylation and that this enzyme catalysed the reaction at high turnover (30/min). 

Betamethasone is well absorbed after oral administration to be extensively bound to plasma proteins in humans, dogs, cows, and rats. Its metabolism does not differ of the other corticosteroids, involving oxidation of the 11 -hydroxyl group to ketone, reduction of the ketone group at the position C-20 to give the corresponding alcohol, and hydroxylation at the C-6 position and loss of the C-17 side chain to give 17-oxosteroids. 

Most forms of Cr(III) are not absorbed and utilized by the body. For this reason, and because of the increased use of sucrose and other refined foods, a marginal human chromium deficiency may be widespread.604 605 This may result not only in poor utilization of glucose but also in other effects on lipid and protein metabolism.597 However, questions have been raised about the use of chromium picolinate as a dietary supplement. High concentrations have been reported to cause chromosome damage606 and there may be danger of excessive accumulation of chromium in the body.607... 

Figure 21-6 Pathways of synthesis and metabolism of sphingolipids. Gray arrows indicate catabolic pathways. See also Fig. 20-11. The green extension on the ceramide structure is that of a long-chain co-hydroxyceramide that is covalently bound to protein in human skin.

A number of procedures used to determine protein quality involve bioassays. Bioassays require feeding live animals protein ingredients for a specified period of time, and then estimating the nutritive value of the protein. Two such assays are the rat-based protein efficiency ratio (PER) bioassay and the human nitrogen balance assay (Dimes et al., 1994). Animal feeding experiments require chemical analyses of both the dietary inputs and then the metabolic output of the animal (e.g., body composition analysis, fecal sample analysis, collection, and assay for urine) from which the efficiency of protein metabolism can be predicted as well as how the protein supports animal growth and cell maintenance. 

Uauy, R. et al., The changing pattern of whole body protein metabolism in aging humans,... 

The endocrine islet cells comprise only 1-2% of the pancreatic tissue. They synthesize the hormones insulin, glucagon, somatostatin, and pancreatic polypeptide. Insulin and glucagon maintain glucose homeostasis via their actions on lipid, carbohydrate, and protein metabolism. The pancreas originates from two patches of epithelium in the duodenum during the fifth week of gestation in humans. The endocrine pancreatic cells begin to differentiate very... 

In humans and animals, chromium(ni) is an essential nutrient that plays a role in glucose, fat, and protein metabolism by potentiating the action of insulin (Anderson 1981). The biologically active form of chromium, called glucose tolerance factor (GTF), is a complex of chromium, nicotinic acid, and possibly amino acids (glycine, cysteine, and glutamic acid). Both humans and animals are capable of converting... 

It has been suggested that BMAA in protein may function as an endogenous neurotoxic reservoir in humans, and may slowly release the neurotoxin directly into the brain during protein metabolism. Incorporation of a nonproternaceous amino acid into a protein may have other serious impacts, such as creating proteins of aberrant function or occurrence [76]. 

Human protein metabolism is essentially dependent on 20 amino acids. These amino acids lend themselves to various classification criteria, which also give hints regarding their biochemical functions (s. tab. 3.6) ... 

Volpi, E., Luddi, P., Crudani, G., Monacchia, F., Santoni, S., Reboldi, G., Brunetti, R, Belli, G.B., de Feo, R Moderate and large doses of ethanol differentially affect hepatic protein metabolism in humans. J. Nutr. 1998 128 198 -203... 

The presence of nucleic acids in yeast is one of the main problems with their use in human foods. Other animals metabolize uric acid to allantoin, which is excreted in the urine. Purines ingested by humans and some other primates are metabolized to uric acid, which may precipitate out in tissue to cause gout (37). The daily human diet should contain no more than about 2 g of nucleic acid, which limits yeast intake to a maximum of 20 g. Thus, the use of higher concentrations of yeast protein in human food requires removal of the nucleic acids. Unfortunately, yields of protein from extracts treated as described are low, and the cost of the protein may more than double. 

Redox reactions and hydrolysis are the predominant metabolic conversions triggered by the intestinal microflora. The primary reductive enzymes produced by the intestinal microflora are nitroreductase, deaminase, urea dehydroxylase, and azoreductase. The hydrolytic enzymes are p-glucuronidase, p-xylosidase, p-galactosidase, and ot-L-arabinosidase. Studies conducted by Macfarlane and coworkers have shown that proteolysis can also occur in the colon. More recent findings by the group indicate that bacterial fermentation of proteins in humans could account for 17% of the short-chain fatty acids in the cecum and for 38 /o in the sigmoid and the rectum. ... 

Shangraw, R. E., Stuart, C. A., Prince, M. J., Peters, E. J., and Wolfe, R. R. (1988). Insulin responsiveness of protein metabolism in vivo following bed rest in humans. Am.. Physiol. 255, E548-E558. 

---