Amino acids, endogenous formation

The rates of formation of various cyclic peptides and DKPs have been documented and shown to be affected by a wide range of physicochemical and structural parameters. Goolcharran and Borchardt examined the effects of exogenous (i.e., pH, temperature, buffer species, and concentration) and endogenous (i.e., primary sequences) factors affecting the rate of cyclic dipeptide formation, using the dipeptide analogues of X-Pro-/)-nitroaniline (X-Pro-/>NA where X represents the amino acid residue of the respective cyclic dipeptide). 

Phase II Reactions. As with phase I reactions, phase II reactions usually depend on several enzymes with different cofactors and different prosthetic groups and, frequently, different endogenous cosubstrates. All of these many components can depend on nutritional requirements, including vitamins, minerals, amino acids, and others. Mercapturic acid formation can be cited to illustrate the principles involved. The formation of mercapturic acids starts with the formation of glutathione conjugates, reactions catalyzed by the glutathione -transferases. 

Subsequent studies by the same group (W7) have shown that the increased endogenous fluorescence in patients with chronic renal failure is due to the unconjugated pteridine, xanthopterin (2-amino-4,6-pteridinedione, 179 Da see Table 4 and Figs. 10 and 11) (B23, Dll, L15, Pll). Unlike the conjugated pteridines (folates), the function of many of the unconjugated pteridines (pterins) has yet to be elucidated (U2, Z3). So far only biopterin has been shown to have a defined role, being a cofactor in the hydroxylation of several aromatic amino acids involved in the formation of neuronal hormones such as catecholamines and serotonin. 

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