Allantoin is a product of adenine and guanine metabolism. Allantoin exists as two enantiomers (R)-(-)-allantoin and (S)-(+)-allantoin that are subject to enzimatic racemization. The spontaneous decomposition of upstream intermediates and the nonenzymatic racemization of allantoin lead to an accumulation of (R)-allantoin, because the enzymes converting allantoin into allantoate are specific for the (S) isomer. The enzyme allantoin racemase catalyzes the reversible conversion between the two allantoin enantiomers, thus ensuring the overall efficiency of the catabolic pathway and preventing allantoin accumulation. The naturally-occurring (+)-allantoin was isolated from leaves of Platanus orientah and from the urine of cattle. (-)-Allantoin was obtained by degradation of the dextrorotatory form of the racemate by allantoinase from soy bean meal or from the liver of Raja clavata.