Methyl donor for 1-methyladenine biosynthesis in starfish ovarian follicle cells

1-Methyladenine (1-MeAde), the oocyte maturation-inducing substance of starfish, is produced by ovarian follicle cells upon stimulation with a gonad-stimulating substance (GSS) released from radial nerves. It has been reported that a process of methylation is involved in GSS-induced 1-MeAde production by starfish ovarian follicle cells. The present study sought to identify a possible methyl donor for 1-MeAde biosynthesis in follicle cells of the starfish Asterina pectinifera. When isolated follicle cells were incubated with [methyl-¹⁴C]methionine (Met), there was an increase in the level of radiolabeled S-adenosylmethionine (SAM). After further incubation with GSS, the [methyl-¹⁴C]SAM level decreased, concomitant with a marked increase in radiolabeled 1-MeAde production. The amount of [methyl-¹⁴C]SAM consumed under the influence of GSS was similar to the amount of [methyl¹⁴C]1-MeAde produced. Therefore, it is concluded that SAM is a methyl donor for 1-MeAde biosynthesis in starfish ovarian follicle cells. On the other hand, it is likely that the purine molecule of 1-MeAde is not derived from SAM but from ATP. 3-Isobutyl-1-methylxanthine, a potent inhibitor of cyclic AMP phosphodiesterase, also caused a reduction in the level of radiolabeled SAM following 1-MeAde production. This suggests that the methylation process of 1-MeAde biosynthesis in starfish ovarian follicle cells upon stimulation with GSS is mediated by a second messenger, cyclic AMP.