All-trans Retinal, also known as Vitamin A aldehyde or Retinaldehyde, is one of the many forms of vitamin A and also the oxidation product of all-trans retinol [1]. All-trans Retinal are associated with one of the two isoforms of cellular retinol-binding proteins (CRBP-I and CRBP-II) with Kd values of 50 and 90 nM, respectively [1].

CRBP-I and CRBP-II were the first intracellular retinoid-binding proteins. Both proteins display a similar binding affinity towards retinal. They play important roles in retinoid biology and regulation of the metabolism of retinol and retinal. CRBP-I is used to regulate vitamin A storage and synthesis of retinoic acid. And CRBP-II has a role in the initial processing of retinol from food [1].

All-trans Retinal is one form of vitamin A. All-trans Retinal, the initial substrate of retinoid cycle, is a chemically reactive aldehyde that can form toxic conjugates with proteins and lipids, leading to degeneration of the retina [2].

References:
[1].? Noy N. Retinoid-binding proteins: mediators of retinoid action. Biochem J. 2000 Jun 15;348 Pt 3:481-95.
[2].? Kiser PD, Golczak M, Maeda A, et al. Key enzymes of the retinoid (visual) cycle in vertebrate retina. Biochim Biophys Acta. 2012 Jan;1821(1):137-51.