EVALUATION OF AN ASPARTAME LOADING TEST FOR THE DETECTION OF HETEROZYGOTES FOR CLASSICAL PHENYLKETONURIA

Classical phenylketonuria (PKU) is an inborn error of metabolism of au tosomal recessive inheritance characterized by the accumulation of phe nylalanine (Phe) in tissues due to Phe4-hydroxylase deficiency. Severa l methods have been developed for the detection of PKU heterozygotes b ased on the determination of plasma Phe and tyrosine (Tyr) levels, on the analysis of the Phe/Tyr and Phe(2)/Tyr ratios and on the use of di scriminant functions. The objective of the present study was to test t he value of loading with aspartame (a sweetener consisting of Phe, asp artate and methanol) for the identification of PKU carriers. The study was conducted on 22 obligate heterozygotes and 27 controls. Two blood samples were collected (under fasting conditions and 30 min after the loading) for fluorometric determination of Phe and Tyr. Phe, Phe/Tyr and Phe(2)/Tyr values were higher in heterozygotes, whereas Tyr was hi gher in controls in both situations investigated. Linear discriminant function was considered to be the best parameter for differentiation o f the individuals in the two groups. Under the conditions employed in the present study, aspartame loading did not show any advantages in di scriminating between PKU carriers and normal individuals when compared to the same analysis performed under fasting conditions.