Visual Pigments and Vitamins A of the Clawed Toad, Xenopus laevis

Abstract

DARTNALL¹ has recently reported extracting what he describes as a new visual pigment from the retina of the South African clawed toad, Xenopus laevis. It is a purple substance, which in Dartnall's preparations has an absorption maximum (λ_max.) at 514–516 mμ. Its difference spectrum—the difference in absorption between it and the products of its bleaching—has λ_max. 521 μ at pH. about 8.6. Dartnall argues, largely on the basis of analogies with the behaviour of other types of preparation, that the absorption maximum of the Xenopus pigment in reality lies at 519 μ. I see no reason, however, for rejecting his measurements, which are as stated. They show the properties of the Xenopus pigment to be intermediate between those of rhodopsin and porphyropsin. The absorption spectrum of rhodopsin ordinarily has λ_max. 500 ± 2mμ, that of porphyropsin λ_max. 522 ± 2 mμ. The Xenopus pigment, with λ_max. about 515 mμ, might on this basis consist of a mixture of about 70 per cent porphyropsin and 30 per cent rhodopsin. Its difference spectrum is similarly intermediate : λ_max. of rhodopsin lies at about 500–502 mμ., that of porphyropsin at about 525–530 mμ. in such mildly alkaline solutions as Dartnall employed; so that again the Xenopus pigment, with difference λ_max. 521 mμ, appears to be a mixture of about two-thirds to four-fifths porphyropsin with one-third to one-fifth rhodopsin.