Coexistence of two pathways of spermidine biosynthesis in lathyrus sativus seedlings

In most organisms, S-adenosyl-L-methionine decarboxylase (SAMDC EC 4.1 .I SO) plays a pivotal role in polyamine biogenesis by contributing S-methyladenosylhomocysteamine which in turn donates its propylamine moiety in a stepwise manner to putrescine and spermidine to yield spermidine and spermine, respectively [l-3]. Two distinct and separable enzymes, namely spermidine synthase (EC 2.5 .l .I 6) and spermine synthase, catalyze these transfer reactions [2]. A distinguishing feature of the SAMDC from higher animals and yeast is its activation by putrescine and to a lesser extent by spermidine, presumably to ensure adequate S-methyladenosylhomocysteamine for enhanced spermidine production [4,5] and hence elevated spermidine/ spermine ratios during growth and development; concurrent inhibition of spermine synthase by the diamine may be a contributory factor in this regulatory phenomenon [4]. While the lack of putrescine activation of the prokaryotic and some lower eukaryotic SAMDCs can be understood in view of near absence of spermine and its synthase in these species [4,6], the presence and augmented production of both the polyamines in the developing higher plants [7,8 J is not easily explained in the absence of the above regulatory mechanism. We reported the purification and some properties of the biosynthetic SAMDC from I,athyrus sativus seedlings and showed that it was of prokaryotic type in being Mg*‘-dependent, but putrescine-insensitive [9]. Both the specific and total enzyme activities were, however, too feeble to account for large in-