RETINAL PHOTOPIGMENTS 133 



was later produced (Detwiler and Zwemer, 1937) which 

 demonstrated that the droplets are Upoidal, staining simi- 

 larly to cephalin. Kolmer (1925) also suggested their lipoidal 

 nature. 



Wald (1935a, 19356) showed that visual purple, when 

 exposed to light, breaks down into retinene (visual yellow) 

 which in turn bleaches further, at room temperature, to 

 form certain colorless products, one of which has been identi- 

 fied as vitamin A. In darkness, at room temperature, retinene 

 recombines to form visual purple. Both retinene and vita- 

 min A are typical lipoids, soluble in organic solvents. Having 

 determined the lipoidal nature of the droplets, we (Detwiler 

 and Zwemer, op. cit.) intimated a possible relation between 

 the droplets, retinene, and vitamin A, rather than with 

 visual purple itself, which has been shown by Wald (19356), 

 and by Hecht, Chase, and Shlaer (1937) to have the nature 

 of a conjugated protein (a carotenoid protein). 



Based upon a careful study of Wald's (1935a, 19356, 1936, 

 1938) analyses of the visual purple cycle, Johnson (1939) sug- 

 gested that the droplets might actually be retinene. 



Recently we (Johnson and Detwiler, 1942) have shown 

 that, whereas the droplets bear at least one property in 

 common with each of the major components of the visual 

 cycle (visual purple, retinene, and vitamin A), they coincide 

 in all respects with only one of these substances, viz., retinene 

 (Table 3). The droplets and vitamin A are both lipoidal 

 and both are absent in extreme vitamin A-deficient animals, 

 but the droplets are most prominent in dark-adapted 

 retinae, whereas vitamin A can be demonstrated in the 

 retina proper only after light-adaptation. Further, the 

 droplets and visual purple show similar behavior under 

 different conditions of illumination, and are both absent in 

 extreme avitaminosis A (Johnson, 1939, Tansley, 1933a), but 

 they differ in essential structure and in mode of concentra- 

 tion. The droplets are lipoidal and demonstrable only in 

 retinae fixed with bichromate-acetic acid solution or in 

 platinum chloride. Visual purple is a conjugated protein 



