Control of Allantoin Accumulation in Comfrey

Abstract

Comfrey, a medicinal herb with healing properties that are attributed to allantoin, was studied in this work. The accumulation and metabolism of allantoin (ALN) and its degradation product, allantoic acid (ALA), were examined. ALN was the main ureide in leaves and roots, with young leaves showing the highest content. ALA was the predominant ureide in the xylem sap, and together with ALN represented 27% of the nitrogen (N) transported in the xylem. Amino acids were the most abundant N-compound in the xylem sap with a high proportion of glutamine. [14C]Xanthine feeding experiments showed that ALN and ALA were actively formed in leaves and roots by degradation of xanthine. Both xanthine and uric acid were rapidly degraded to form ALN and ALA. Enzyme studies showed that calculated V max /K m are low for allantoinase and alantoicase, supporting the results from the feeding experiments, and indicating that accumulation of ALN in comfrey is due to a low capacity for the enzymatic degradation of ureides.