The synthesis of sulphatoxygalactosylacylalkylglycerol (SGG) is a differentiation marker of mammalian spermatogenesis. Maximal sulphation is observed in rat testis at about 20 days after birth and rapidly declines to low levels as the testis matures. The present data show that this decline in SGG synthesis is due to the appearance of an inhibitor of galactolipid sulphation. The inhibitor is a soluble testicular factor which is first detected at about 25 days after birth. Testicular homogenate can sulphate exogenous galactosylacylalkylglycerol (GG), galactosylceramide (GC) and lactosylceramide (LC) in vitro. The testicular inhibitor is most effective in preventing GG sulphation and inhibits GC and LC sulphation to a lesser extent; this correlates with the finding that glycolipid sulphation shifts from SGG production in 20-day-old testis to GC and LC sulphation at later stages of testicular development. The effect of the inhibitor on sulphotransferase activity from brain and kidney was also determined. The inhibitor decreased the sulphation of GG in vitro by both testis and kidney, inhibited testicular sulphation of GC less effectively and had no effect on GC sulphation by kidney and brain homogenates. A 9500-fold purification of the inhibitory activity has been obtained in a fraction isolated by h.p.l.c.

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