In cells, all- trans -retinol can be either stored (in the form of retinyl ester) or oxidized to all- trans -retinal by alcohol dehydrogenases.
Retinaldehyde dehydrogenases can catalyze the conversion of all- trans -retinal into two biologically active isomers of retinoic acid (RA): all- trans -RA and 9- cis -RA.