The acrosome reaction is accompanied by ionic changes such as increases in intracellular Ca2+ and intracellular pH (pHi). Since the two jelly components essential for inducing the acrosome reaction, ARIS and Co‐ARIS, were shown to activate Ca‐channels (accompanying paper), we examined the jelly components to determine which was responsible for the pHi‐increase using 9‐aminoacridine as a probe of pHi. This paper presents evidence that an oligopeptide(s) is responsible for the pHi‐increase. The pHi of swimming sperm is 7.4‐7.5. Within 20 sec after the addition of jelly, their pHi increased rapidly by 0.06 pH unit, then decreased by 0.2–0.3 pH unit, and reached a plateau level within 3 min. Similar changes in pHi were observed on addition of a Pronase digest of ARIS (P‐ARIS) and a diffusible fraction of jelly (Fraction M8) together. Fraction M8, but not ARIS or Co‐ARIS increased the pHi, and activated sperm respiration in sea water at pH 6.5. The two activities of Fraction M8 depended upon Na+ but not Ca2+, and were susceptible to Pronase digestion. Fraction M8 is also known to enhance induction of the acrosome reaction by the Ca‐ionophore A23187. These results suggest that the egg jelly contains a peptide(s) that is not obligatory for the acrosome reaction but facilitates the reaction by increasing the pHi of the sperm. The significance of the pHi‐increase upon the acrosome reaction is discussed.
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