Two components in the egg jelly are required for inducing the acrosome reaction in starfish; a sulfated glycoprotein called acrosome reaction‐inducing substance (ARIS) and its cofactor called Co‐ARIS. Three distinct molecules were isolated as the major Co‐ARIS' and designated as Co‐ARIS' I, II and III. Structural analysis of Co‐ARIS' revealed that they are steroidal saponins comprising a sulfated steroid and a pentasaccharide chain. Co‐ARIS' I and II differ only in the steroidal side chain. In the presence of ARIS, each Co‐ARIS induced the acrosome reaction with a maximal effect at 100–200 μM (Co‐ARIS I) or 25–50 μM (Co‐ARIS' II and III). Mixtures of Co‐ARIS' I, II and III were more effective than the individuals. The activity of Co‐ARIS was considerably reduced by solvolytic desulfation but was not affected at all by periodate oxidation. Reduction by NaBH4 decreased the activity of Co‐ARIS I and enhanced that of Co‐ARIS II. Treatment of Co‐ARIS III with NaBH4 did not affect the activity as anticipated from its structure. These results suggest that the sulfate moiety and the side chain of steroid are important for the activity of Co‐ARIS. The saccharide chain, however, seems not necessarily to be strictly specified for the activity.
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