The metabolic conversion of phenylalanine into tyrosine in the human kidney: does it have nutritional implications in renal patients?

Recent evidence provided by the in vivo measure of the activity of phenylalanine hydroxylase in humans indicates that the kidney plays a role greater than previously presumed in phenylalanine conversion to tyrosine, an amino acid which has been considered nonessential so far. Homeostasis of tyrosine pools is only partially restored by a reduced uptake of the same amino acid by splanchnic organs in the well-nourished noncatabolic patient with chronic renal failure. Tyrosine pools in uremia can also be restored by an increase in endogenous net protein catabolism, because it occurs during treatment with nonbiocompatible membranes or during acidosis. However, these are trade-offs that are associated with a progressive decrease in muscle mass. Based on these findings, one can argue that with progressively declining renal function and kidney metabolic activity, the nutritional requirements for tyrosine increase progressively. This mechanism could in part account for the increased protein requirements in dialysis-treated end-stage renal disease patients, as compared with predialysis patients.