# ALDH1A1

Aldehyde dehydrogenase 1 family, member A1, also known as ALDH1A1 or retinaldehyde dehydrogenase 1 (RALDH1), is an enzyme that is encoded by the ALDH1A1 gene.

## Function

This protein belongs to the aldehyde dehydrogenases family of proteins and is a member of the ALDH1 subfamily (including ALDH1A2, ALDH1A3, ALDH1B1, ALDH2). Aldehyde dehydrogenase isozymes are NAD(P)-dependent dehydrogenases that catalyze the oxidation of an aldehyde into the corresponding carboxylic acid while reducing NAD+ or NADP+. ALDH1A1 is the only ALDH1 isozyme known to oxidize 9-cis retinaldehyde into 9-cis retinoic acid and thus serve as the only known activator of the rexinoid nuclear receptor pathway. ALDH1A1 has also been described with activity against other substrates in living systems, including all-trans retinaldehyde as well as oxazaphosphorine, a cyclophosphamide metabolite. Unique among the ALDH1 isozymes, ALDH1A1 is known to possess esterase activity in biochemical studies, although it is unclear whether this is functionally relevant living tissues.
ALDH1A1 is expressed predominantly in metabolic tissues, including the liver, gastrointestinal tract, thyroid, pituitary gland, and adipose tissues. ALDH1A1 is also expressed in the testes where its function in spermatogenesis is subordinate to and compensatory for ALDH1A2 in mice. ALDH1A1 is inhibited by Antabuse (disulfiram), though the primary pharmacologic target of disulfiram in clinical use is ALDH2. The long clinical history of disulfiram use suggests that ALDH1A1 is not important to normal human physiology. Tumors, specifically in ovarian cancer are found to have a high expression of ALDH1A1.ALDH1A1 is found to cause resistance to chemotherapy.

## Clinical significance
### Obesity

The removal of ALDH1A1 in mice through genetic knockout results in viable animals that are fertile and healthy. The only validated phenotype of these mice is a resistance to high fat diet-induced obesity while whole body ALDH1A1 removal does not affect fertility or neurological function. This biology closely replicates the clinical profile of Antabuse (disulfiram). Disulfiram and other ALDH1A1 inhibitors have been shown to cause ALDH1A1-dependent weight loss in obese animals. This has increased interest in disulfiram as an alternative weight loss therapy to Ozempic, yet the rare but potentially fatal liver-damaging effects of disulfiram due to its broad lack of selectivity as well as the alcohol-disulfiram reaction make it unattractive as a weight loss therapy. Subsequent efforts to produce ALDH1A1-specific inhibitors have resulted in preclinical compounds that induce weight loss through increased metabolic activity.

## Errors due to historical nomenclature

ALDH1A1 is often attributed with multiple biological roles as studies prior to human genome sequencing operated under the assumption that only one ALDH1 gene existed rather than the five isozymes that are annotated today. Accordingly, ALDH1A1 is often attributed with a role in alcohol metabolism through oxidation of acetaldehyde, however, single nucleotide polymorphisms (SNPs) in this enzyme show little evidence of linkage to alcoholism in humans. Despite established naming conventions, many studies still incorrectly use ALDH1 to describe the family of isozymes. For instance, many cancers studies have been interpreted to report on ALDH1A1 activity when the actual protein was ALDH1A3.

## Species-specific expression

ALDH1A1 possesses unique taxon-specific traits across mammals. Found uniquely in rabbits compared to other mammals, ALDH1A1 appears to function as a corneal crystallin that helps to maintain the transparency of the cornea. In other species such as humans, this role is performed by ALDH3A1. In beavers, the ALDH1A1 gene has undergone genomic expansion, resulting in approximately 10 copies of the genomic locus, which is putatively linked to a role in lipid balance.