Maturation-Inducing Substances in Asteroid and Echinoid Oocytes

In starfish a hormonal peptide, gonad-stimulating substance (GSS), which is released from nervous tissue, acts on the gonad to produce a maturation-inducing substance (MIS), an inducer of oocyte maturation and spawning. MIS is 1-methyladenine (1-MA). This substance acts on the surface of the oocyte. Cytoplasmic maturation as revealed by fertilizability is also induced by 1-MA. The amount of 1-MA can be determined very accurately with a bioassay method using isolated oocytes. In 1-MA formation, GSS seems to enhance the methylation of some compound which contains a purine nucleus at its N1 site. The methyl donor is probably S-adenosylmethionine. 1-Methylated precursor seems to be transformed to 1-methyl AMP and then hydrolyzed into 1-methyladenosine and phosphate by phosphomonoesterase. 1-Methyladenosine is finally split into 1-MA and ribose by 1-methyladenosine ribohydrolase. So-called spontaneous maturation of oocytes isolated in sea water is due to the action of 1-MA produced in follicle cells even in the absence of GSS. 1-MA is present in echinoid gonads and seems to act as MIS also in these animals. Disulfidereducing agents such as dithiothreitol and 2,3-dimercapto-1-propanol induce starfish oocyte maturation. On the other hand, sulfhydryl reagents such as p-chloromercuribenzoate, iodoacetamide, and N-ethylmaleimide suppressed 1-MA-induced oocyte maturation. Since Concanavalin A acts on the follicle cells to produce 1-MA, the action of this substance seems to be quite similar to that of GSS.