DHRS3 (Short-chain dehydrogenase/reductase 3; retSDR1 / RDH17 / SDR16C1) — review notes

UniProt: O75911 (DHRS3_HUMAN), 302 aa, HGNC:17693, gene on chromosome 1.

Core biochemistry

DHRS3 is a microsomal, membrane-bound short-chain dehydrogenase/reductase (SDR) that
catalyzes the NADPH-dependent reduction of all-trans-retinaldehyde to all-trans-retinol.

UniProt FUNCTION: "Catalyzes the reduction of all-trans-retinal to all-trans-
retinol in the presence of NADPH." [file:human/DHRS3/DHRS3-uniprot.txt]
UniProt CATALYTIC ACTIVITY: "Reaction=all-trans-retinol + NADP(+) = all-trans-retinal + NADPH +
H(+); ... EC=1.1.1.300; Evidence={ECO:0000269|PubMed:9705317}" [file:human/DHRS3/DHRS3-uniprot.txt]
This maps exactly to GO:0052650 all-trans-retinol dehydrogenase (NADP+) activity
(def: "all-trans-retinol + NADP+ = all-trans-retinal + NADPH + H+"; OLS-verified MF).
The enzyme uses NADPH, not NADH; the IBA GO:0004745 "all-trans-retinol dehydrogenase
(NAD+) activity" therefore has the wrong cofactor and should be MODIFIED to GO:0052650.
Cofactor: UniProt KW "NADP"; a classical Rossmann fold for nucleotide binding was predicted
by homology modeling: "predicted a classical Rossmann fold for the nucleotide binding"
PMID:9705317. Supports GO:0050661 NADP binding as the specific cofactor-binding MF
(the existing GO:0000166 nucleotide binding is the uninformative parent).

Original characterization (PMID:9705317, Haeseleer et al. 1998; abstract only, full_text_available: false)

Cloned "retSDR1", an SDR that reduces all-trans-retinal; "localizes predominantly in cone
photoreceptors" and "retSDR1 expressed in insect cells displayed substrate specificities of
the photoreceptor all-trans-retinol dehydrogenase." PMID:9705317
"Homology modeling of retSDR1 using the carbonyl reductase structure as a scaffold predicted
a classical Rossmann fold for the nucleotide binding, and an N-terminal extension that could
facilitate binding of the enzyme to the cell membranes." PMID:9705317
"The presence of retSDR1 in a subset of inner retinal neurons and in other tissues suggests
that the enzyme may also be involved in retinol metabolism outside of photoreceptors."
PMID:9705317 — i.e. the retina/visual role is only one context; retinol metabolism is the
general function.
The GOA TAS annotations attributed to this paper for GO:0009055 electron transfer activity
and GO:0000166 nucleotide binding are legacy over-general mappings: the abstract describes a
retinaldehyde reductase (hydride transfer to/from NADP), not a dedicated electron carrier,
and "nucleotide binding" is the uninformative parent of NADP binding.

Role in retinoic-acid homeostasis and development (PMID:40519748, Hashimoto et al. 2025; full_text_available: true)

DHRS3 opposes the retinaldehyde dehydrogenases: "dehydrogenase/reductase 3 (DHRS3, also named
SDR16C1; encoded by DHRS3, HGNC:17693) is the major embryonic enzyme catalyzing the opposite
reaction (RAL to ROL)." PMID:40519748
Physiological role = buffering RA synthesis; the paper's abstract frames DHRS3 as "limiting
excessive RA synthesis." PMID:40519748 Loss of DHRS3 therefore raises RA:
"Cells transfected with a DHRS3-Val171Met construct exhibited reduced retinaldehyde reduction
capacity compared with wild-type, yielding reduced retinol and elevated RA" PMID:40519748.
Mutually activating complex with RDH10 in the ER membrane: "RDH10 and DHRS3 function together
as a molecular complex inserted within the membrane of the endoplasmic reticulum." PMID:40519748
This supports the ER-membrane localization (GO:0005789).
Biallelic hypomorphic DHRS3 variants cause a novel RA embryopathy (coronal craniosynostosis,
facial dysmorphism, congenital heart disease, scoliosis); mouse Dhrs3-/- is embryonic-lethal
with ~40% increased embryonic RA. This is the IMP evidence (ECO:0000315) for the GOA
annotation GO:0002138 retinoic acid biosynthetic process (involved_in). Because DHRS3's direct
action drains the retinaldehyde pool and thus down-regulates RA synthesis, its participation
in the RA pathway is regulatory/negative; GO:1900053 negative regulation of retinoic acid
biosynthetic process is a biologically more precise term for the developmental phenotype,
but per policy the experimental IMP annotation is retained (not removed).

Retinoic-acid inducibility and expression (PMID:11861404, Cerignoli et al. 2002; not cited in GOA seed)

UniProt INDUCTION: "By retinoic acid." [file:human/DHRS3/DHRS3-uniprot.txt] — this makes DHRS3
a RA-inducible negative-feedback buffer on RA levels.
UniProt TISSUE SPECIFICITY: "Widely expressed ... In the retina, expressed in cone but not rod
outer segments." [file:human/DHRS3/DHRS3-uniprot.txt]
UniProt MISCELLANEOUS: located in a chromosome-1 region often deleted in aggressive
neuroblastoma. [file:human/DHRS3/DHRS3-uniprot.txt]

Subcellular localization

UniProt SUBCELLULAR LOCATION: "Membrane {ECO:0000305}; Multi-pass membrane protein
{ECO:0000305}." [file:human/DHRS3/DHRS3-uniprot.txt] Four predicted TM helices (FT TRANSMEM
9..29, 170..190, 195..215, 253..273).
ER membrane is the informative curated location (Reactome R-HSA-5419165; and the RDH10/DHRS3
complex is ER-membrane-inserted per PMID:40519748). GO:0016020 membrane (IEA subcell mapping)
is correct but is the general parent of GO:0005789 endoplasmic reticulum membrane.
Reactome R-HSA-2465940 places the atRAL->atROL reduction step at the photoreceptor outer
segment membrane in the visual (retinoid) cycle; retina-specific, non-core.

Annotation decision summary

Term	Evid	Action	Rationale
GO:0052650 all-trans-retinol dehydrogenase (NADP+) activity	EXP/TAS/IEA	ACCEPT (core MF)	matches EC 1.1.1.300 / UniProt reaction
GO:0004745 (NAD+) activity	IBA	MODIFY -> GO:0052650	wrong cofactor (enzyme is NADPH-dependent)
GO:0000166 nucleotide binding	TAS	MARK_AS_OVER_ANNOTATED	uninformative parent; better = GO:0050661 NADP binding
GO:0009055 electron transfer activity	TAS	MARK_AS_OVER_ANNOTATED	reductase, not electron carrier
GO:0001523 retinoid metabolic process	IBA/TAS	ACCEPT	correct pathway context
GO:0042572 retinol metabolic process	TAS	ACCEPT (core BP)	direct product/substrate
GO:0002138 retinoic acid biosynthetic process	IMP	KEEP_AS_NON_CORE	experimental; DHRS3 negatively regulates RA synthesis
GO:0048385 regulation of RAR signaling	IBA	KEEP_AS_NON_CORE	downstream of RA-level control
GO:0007601 visual perception	TAS	KEEP_AS_NON_CORE	retina/cone context only
GO:0005789 endoplasmic reticulum membrane	TAS	ACCEPT (core loc)	RDH10/DHRS3 ER complex
GO:0016020 membrane	IEA	KEEP_AS_NON_CORE	correct but general parent of ER membrane
GO:0005811 lipid droplet	IBA	KEEP_AS_NON_CORE	family-level; plausible for lipid-metabolizing SDR
GO:0042622 photoreceptor outer segment membrane	TAS	KEEP_AS_NON_CORE	retina-specific visual-cycle context

Core functions

MF: GO:0052650 all-trans-retinol dehydrogenase (NADP+) activity (retinaldehyde reductase, EC 1.1.1.300)
MF: GO:0050661 NADP binding (cofactor)
BP: GO:0042572 retinol metabolic process / GO:0001523 retinoid metabolic process; regulatory role in retinoic-acid homeostasis (buffering RA synthesis, essential in development)
CC: GO:0005789 endoplasmic reticulum membrane