Saliva and serum neopterin concentrations not significantly correlated in HIV-1 infection.

950 CLINICAL CHEMISTRY, Vol. 41, No. 6, 1995 creasing concentrations of these amino acids were analyzed (data not shown). These results suggest that Tyr and Leu in serum are measured as Phe to various degrees by the PDH preparations examined here. The increased utilization of Tyr and Leu by the Sigma and Biocatalysts preparations of PDH accounts for the positive biases obtained with these PDH sources. The Calbiochem and Biocatalysts PDH are prepared from Rhodococcus, whereas Sigma PDH is isolated from Sporosarcina ureae. A previous report found substantial reaction of other amino acids with PDH from S. ureae (3), which may explain the reaction with Tyr and Leu by the Sigma PDH preparation seen in our studies. The differences in utilization of Tyr and Leu between the Biocatalysts and Calbiochem PDH preparations might reflect different degrees of purity, such that contaminating activity in the Biocatalysts preparation oxidizes amino acids other than Phe. Of the three sources, only Biocatalysts makes a claim in regard to the specificity of the PDH, stating that reaction with Tyr is <0.5% of that with Phe. Even this low extent of reaction with Tyr can result in substantial interference in end-point clinical assays, where the excess PDH activity added to assure timely, complete reaction also enhances the opportunity of contaminating activity to catalyze side reactions. We urge further effort on the part of manufacturers of PDH to remove contaminating activities or utilize bacterial sources of PDH with minimal cross-reactivity with other amino acids. Until preparations become available that are aimed at (and assessed for) clinical application, users must evaluate independently the actual activity and specificity of PDH preparations they wish to employ.