LARGE DAILY FLUCTUATIONS IN PLASMA TYROSINE IN TREATED PATIENTS WITH PHENYLKETONURIA

In patients with phenylketonuria (PKU), extra tyrosine supplementation is advocated in addition to tyrosine-enriched amino acid mixtures. PK U patients have low fasting plasma tyrosine concentrations, but little is known about tyrosine fluctuations during the day. Plasma tyrosine concentrations were studied in 12 PKU patients in response to a test w ithout breakfast and to three tests with different tyrosine contents i n breakfast and lunch: 0%/30%, 25%/30%, 50%/10%, and 75%/10% tests, re flecting the protein consumption at breakfast and lunch, respectively. Prolonged fasting resulted in a small decrease in the already low ove rnight fasting plasma tyrosine concentrations. Breakfast and lunch wit h 25% and 30% of the daily tyrosine intake resulted in both lower than normal and higher than normal tyrosine concentrations. The 50%/10% an d 75%/10% tests resulted in excessively high plasma tyrosine concentra tions in most patients. Therefore, both lower than normal and higher t han normal postprandial plasma tyrosine concentrations were found in t reated PKU patients, even if the daily tyrosine intake was distributed evenly. When there was a large fractional tyrosine intake from one me al, very high plasma tyrosine concentrations were found. Therefore, st rict control of plasma tyrosine is necessary if tyrosine supplementati on is considered in addition to the tyrosine-enriched amino acid mixtu res.