Injection of estrogen into male Xenopus laevis induced the appearance of retinals (retinal and 3-dehydroretinal) and a considerable increase in the amount of retinols (retinol and 3-dehydroretinol) in the blood plasma. These retinoids were mainly in the all-trans form. Without estrogen injection, retinols were normally found in the blood plasma of both males and females, but only trace amounts of retinals were detected and these were restricted to the plasma of females. The proteins in the blood plasma of estrogen-injected males were separated into two fractions. One fraction included vitellogenin, the precursor of egg yolk proteins, and the other contained some plasma proteins other than vitellogenin. Retinals were detected in the former and retinols in the latter. It is suggested that retinals are bound to vitellogenin and are taken up into oocytes in the process of vitellogenesis.

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