Metabolism of the retina. III. The role of reduced triphoshopyridine nucleotide in the visual cycle.

Exposure of a dark-adapted retina to light results in the liberation of vitamin A aldehyde (retinene) from the visual pigment, rhodopsin, and in subsequent enzymatic reduction of the liberated aldehyde to vitamin A (1). The enzyme, alcohol dehydrogenase (retinene reductase), is present in visual cell outer segments and can use reduced diphosphopyridine nucleotide (DPNH) for the reduction of vitamin A aldehyde (2). Observations that crystalline liver alcohol dehydrogenase could reduce vitamin A aldehyde to vitamin A (3) and that this enzyme was virtually incapable of functioning with TPN (4) were sufficient to establish the concept that in the visual cycle DPNH reduces vitamin A aldehyde to vitamin A. Recently liver alcohol dehydrogenase was purified by a new procedure and was shown to be capable of functioning with TPN (5). It is clear, therefore, that the pyridine nucleotide requirements of processes involving enzymatic reduction of aldehydes to alcohols should be reinvestigated. Evidence is presented in this report that TPNH rather than DPNH is the principal reducing agent for vitamin A aldehyde in the visual cycle of the retina.