The Role of Transamination in the Synthesis of Homoserine in Peas

Abstract

Incubation of intact pea plants (Pisum sativum), or detached shoots, in continuous light caused a substantial increase (up to 4-fold in 2 days) in levels of homoserine. Amino acids supplied to leaves in the transpiration stream enhanced the accumulation, with glutamate, aspartate, and asparagine causing similar enhancement. Aminooxyacetate (AOA), a transamination inhibitor, at 1 millimolar prevented the accumulation. ¹⁴C-labeling experiments showed that succinate was a good source of carbon for homoserine synthesis; carbon from aspartate or asparagine was also incorporated into homoserine. For each precursor, the transfer of label was prevented by AOA. The keto acid analog of homoserine was rapidly transaminated in leaves to give homoserine. The results suggest that accumulating homoserine is synthesised by transamination rather than being derived from aspartate via the aspartate kinase/homoserine dehydrogenase pathway. The latter pathway was shown to be operating in the chloroplasts, and was sensitive to threonine (but was not inhibited by AOA), suggesting that this path has a role in synthesis of aspartate-derived amino acids but is not involved in the accumulation of excess homoserine in the pea.