Plasma Albumin Redox State Is Responsive to the Amino Acid Balance of Dietary Proteins in Rats Fed a Low Protein Diet

Abstract

We recently reported that plasma albumin redox state, which correlates with albumin synthesis rate, could be associated with the quality of dietary protein. Aiming to elucidate the association between them, plasma albumin redox state was investigated in rats fed various kinds of AIN-93G-based low protein diets. Plasma albumin redox state was shifted to a more oxidized state in rats fed 3% casein (CN) diet than those fed 3% whey protein or 3% wheat gluten diet, while supplementing 3% CN diet with cystine reversed it to a more reduced state, indicating that cystine would complement the shortage of cysteine in CN, thereby increasing albumin synthesis rate. Supplementation with glutathione, a cysteine-containing antioxidative tripeptide, normalized hepatic glutathione redox state modulated by ingestion of 3% CN diet, but it only reversed the oxidized shift of plasma albumin redox state to an extent similar to cystine alone or the constituting amino acid mixture of glutathione (i.e., glutamic acid, cystine, and glycine), indicating that glutathione would primarily serve as a source of cysteine rather than exert its antioxidative activity. Plasma albumin would thus be influenced by amino acid balance in dietary proteins, and it could be useful as a biomarker that contributes to prevention of protein under-nutriton, caused by not only insufficient protein intake but also ingestion of poor-quality protein.

Serum Albumin Redox States: More Than Oxidative Stress Biomarker

“…However, a series of recent animal studies have demonstrated that nutritional factors also modulate ALB redox state. Plasma ALB redox state shifted to a more oxidized state in rats fed a low-protein diet [ 25 , 26 , 90 , 91 , 92 , 93 ]. This shift was mediated by decreased ALB turnover including hepatic ALB synthesis rate [ 91 ], and was independent of oxidative stress as examined by plasma thiobarbituric acid reactive substance (TBARS) and advanced oxidation protein product (AOPP) concentration [ 26 , 91 ].…”

“…This shift was mediated by decreased ALB turnover including hepatic ALB synthesis rate [ 91 ], and was independent of oxidative stress as examined by plasma thiobarbituric acid reactive substance (TBARS) and advanced oxidation protein product (AOPP) concentration [ 26 , 91 ]. ALB redox state was also responsive to amino acid balance in diet [ 92 , 93 ]. Furthermore, plasma ALB redox state responded to a low-protein diet ingestion more sensitively compared with plasma ALB concentrations, a conventional protein nutrition biomarker [ 25 , 26 ].…”

“…HPLC Increase in HNA-1 [25,26,[90][91][92][93] * Thiols binding to total plasma proteins. Cys, cysteine; HCys, homocysteine; and HNA, human non-mercaptalbumin.…”

Cystine supplementation sustains plasma mercaptalbumin levels in rats fed low-protein diets more effectively than methionine

“…A recent study showed that supplementation with glutathione to a low-protein diet only reversed the percentage of MA within total plasma albumin to an extent similar to that observed with Cyss supplementation. ( 36 ) The authors of that study suggested that the glutathione supplemented to the low-protein diet would primarily serve as a source of Cys rather than exert antioxidative activity. The percentage of MA within total plasma albumin might be regulated by dietary Cyss via the glutathione-independent pathway.…”