Labilization of the α-Hydrogen Atom of Generally Labelled Tritiated L-α-Amino-acids in the Presence of Renal D-Amino-acid Oxidase

Abstract

WE have found that the specific activity of L-amino-acids labelled with tritium, which were isolated after D-amino-acid oxidase treatment of the corresponding generally labelled DL-compound, was much lower than expected. The DL-amino-acids were labelled by exchange with tritiated water in the presence of a platinum catalyst¹, and reverse isotope dilution analysis with both D- and L-carriers² showed that the tritiated amino-acids were labelled in the D- and L-components to the same extent. Paper chromatographic analysis³ confirmed the radio-chemical purity (> 98 per cent) of the DL-amino-acids, which were afterwards treated with crude D-amino-acid oxidase (hog kidney extract) by well-established techniques⁴. The specific activity of the tritiated compounds was measured by combustion of the compounds in oxygen followed by β-liquid scintillation counting of the tritiated water produced³. The radiochemical purity of the isolated L-amino-acids was determined by paper chromatographic analysis and the optical purity by reverse isotope dilution analysis with both L- and D-carriers. If no labilization of the tritium atoms had occurred, after the D-oxidase treatment the isolated L-isomer should thus have the same specific activity as the parent DL-compound. The results shown in Table 1 illustrate that this is not the case and that there is a substantial loss of tritium from the L-isomer.