Rdh11, also known as SCALD, is an NADPH-dependent retinaldehyde reductase. It is involved in multiple biological processes, including retinoid metabolism, sex steroid synthesis, and cholesterol metabolism. It has been suggested as a tentative HSD17B (HSD17B15) in humans and may play a role in the visual cycle, a pathway crucial for vision [1, 4-7, 9].

In gene knockout studies, Rdh11-/-mice show delayed dark adaptation, indicating its role in retinoid metabolism in photoreceptor inner segments [3]. However, the kinetics of 11-cis-retinal recycling during dark adaptation was not affected, suggesting it may not be directly involved in the visual cycle. In Japanese eel, three cDNAs related to human RDH11 were isolated, and the recombinant Rdh11/12-like proteins metabolize sex steroids, with Rdh11/12-like 1 potentially involved in 11KT production in the testis [1]. In addition, in mice, hepatic Rdh11 mRNA expression is regulated by diet and statin treatment, and modulation of RDH11 affects cholesterol metabolism, inflammation, and cellular stress pathways [2].

In conclusion, Rdh11 plays essential roles in retinoid and sex steroid metabolism and in maintaining cellular cholesterol levels. Gene knockout mouse models have revealed its functions in dark adaptation, sex steroid synthesis, and cholesterol-related stress response, contributing to our understanding of vision-related diseases, reproductive disorders, and cardiovascular diseases [1,2,3].