Saccharopine: an Intermediate of L-Lysine Biosynthesis and Degradation in Pyricularia oryzae

Abstract

The aminoadipic acid pathway has previously been found to function as a biosynthetic route to lysine in higher fungi and as a catabolic pathway in mammals and higher plants. Results are now presented which suggest that in Pyricularia oryzae (fungi imperfecti), the aminoadipic acid pathway is important in both the biosynthesis and catabolism of lysine. In vivo experiments using L-[4-14C]aspartate showed that 98% of the radioactivity in protein-bound lysine and aspartate was in the latter, and with dl-amino[1-14C]adipic acid radioactivity was readily incorporated into saccharopine and free and protein-bound lysine. Endogenously supplied aminoadipic acid and saccharopine caused a rapid and substantial rise in the concentrations of free lysine in the mycelium. Cell-free extracts catalysed the conversion of aminoadipic acid to saccharopine and of saccharopine to lysine. Pyricularia oryzae and several other fungi were found to contain significant quantities of the intermediate saccharopine. The concentration of saccharopine increased when P. oryzae was supplied with lysine, and mycelia converted L-[14C]lysine into saccharopine and 2-amino-adipic acid. In addition, all of the enzyme activities necessary to break lysine down to 2-oxoadipic acid were demonstrated in extracts.