Cytokinin Activity of a Group of Ureidopurine Derivatives Related to the tRNA Component, N-(Purin-6-ylcarbamoyl)threonine

Abstract

Several ureidopurine derivatives have been tested for cytokinin activity in a soya bean callus assay system. N-(Purin-6-ylcarbamoyl)threonine (Ade-CO-Thr), a naturally occurring component of tRNA, is inactive in the assay. Analogues of Ade-CO-Thr having unmodified hydrocarbon side chains are active; optimal activity is associated with tert-butylureidopurine. In the homologous straight chain series n-butylureidopurine is the most active. Introduction of −COOH and/or −OH groups to analogues with otherwise active alkyl chains negates activity.The phenylureidopurine (PUP) derivative is barely active itself, though ortho-halogenated derivatives (o-chloro-, o-fluoro-PUP) are the most potent ureidopurines known. Biological activity decreases rapidly as the electronegative halide is moved from the ortho to the meta position, and lost entirely in the para position. Replacement of the ortho halide with a non-electronegative group (methyl) greatly reduces activity. Molecular models reveal that Ade-CO-Thr and o-chloro-PUP have nearly identical side-chain bulk. The carboxyl groups of Ade-CO-Thr and the chlorine atom of o-chloro-PUP occupy the same position relative to the electronegative carboxyl oxygen of the urea bridge. It is proposed that Ade-CO-Thr has a growth regulatory role in the intact organism, but that exogenous Ade-CO-Thr may be unable to reach the active site.

Synthesis and biological activity of analogs of naturally occurring 6-ureidopurines and their nucleosides

“…"isocyanate of glycine methyl ester was allowed to react with tri-O-acetyladenosine. lC: caled, 45.45; found, 43.05. mThis compound was reported by McDonald, et al 21 "In 95% EtOH solution (0.1 N NaOH). °N: caled, 20.53; found, 21.48 (9) and glycine ethyl ester (10).…”

“…PCT has been found in tobacco tRNA;20 however, it had no cytokinin activity in soybean as well as in tobacco tissue test systems. 21 The PCTR, PCG, PCGR, and their analogs containing other natural amino acids were also inactive as cytokinins.21 These compounds containing free acidic groups were also inactive in mammalian cell lines when tested for their growth inhibitory activity.5 6-Allylureidopurine21 and its ribonucleoside 74, which can be looked upon as a metabolite of PCTR, exhibited good cytokinin activity; however, they showed only marginal growth inhibitory activity in the mammalian cell lines. 6-n-Butylureidopurine21 and the 6-isopropylureidopurine (18) were also good as cytokinins but were inactive as mammalian cell growth inhibitors.…”

Cytokinins in Rhizopogon roseolus Secretion of N-[9-(β-d-Ribofuranosyl-9H) Purin-6-Ylcarbamoyl]Threonine into the Culture Medium

“…Some of these analogs also exert activity in the tobacco callus assay system (14). These results led Dyson et al (9) to suggest that Ade-CO-thr is the archetype structure of this series of analogs.…”

“…In view of the potent cytokinin activity of N6-(A2-isopentenyl)adenosine, Dyson et al (9) studied the question whether Ado-CO-thr has cytokinin activity. Neither Ado-COthr nor its corresponding base, Ade-CO-thr, exerts activity in the tobacco callus or soybean assay systems.…”