Plasma amino acid values and pancreatic beta-cell function in phenylketonuria.

In 16 phenylketonuric (PKU) patients aged 5-12 years, plasma glucose, immunoreactive insulin (IRI), C-peptide (CP) and plasma amino acids were measured in basal conditions and under a standard oral glucose tolerance test (OGTT). The beta-cell response to OGTT was higher in PKU patients than in normal subjects as demonstrated by peak levels and areas under the curves of plasma concentrations of IRI and of CP. A significant correlation was observed between plasma phenylalanine values and both IRI and CP 'output' in PKU patients. Mean concentrations of branched chain amino acids and tyrosine in plasma decreased significantly during OGTT, while phenylalanine values increased in PKU subjects.