A role for leaf peroxisomes in the catabolism of purines

The presence of purines and allantoin in peroxisomes purified from pea (Pisum sativum L.) leaves was studied. Purified intact peroxisomes were pretreated to remove proteins, and analyzed by an HPLC method to identify and quantify the levels of the metabolites xanthine, hypoxanthine, uric acid, and allantoin in leaf peroxisomes. Uric acid and xanthine were clearly detected in leaf peroxisomes, 20 mu g of xanthine and 21 mu g of uric and per mg peroxisomal protein. Allantoin also seemed to be present in peroxisomes, but hypoxanthine was not identified. The existence of allantoin in peroxisomes was verified enzymatically by determining the concentration of this ureide with commercial allantoinase, and an amount of about 7 mu g allantoin per mg peroxisomal protein was measured. In purified intact peroxisomes xanthine oxidase and urate oxidase activities were found to be present, but allantoinase activity was not detected. These results show for the first time that purines and allantoin are present in leaf peroxisomes and that xanthine can be catabolized to uric acid and allantoin inside these organelles. The N-rich ureide allantoin could be further used by leaf cells for synthesis of amino acids and proteins.