Inter-individual variability, differential tissue abundance, and sub-cellular localization of human aldo-keto reductases (AKRs) and hydroxy-steroid dehydrogenases (HSDs).

Human aldo-keto reductases (AKRs) and hydroxysteroid dehydrogenases (HSDs) are oxidoreductases involved in the metabolism of steroidal hormones and xenobiotics; however, their inter-individual variability, tissue abundance, and subcellular localization remain understudied. This study employed quantitative proteomics to address these knowledge gaps utilizing cryopreserved human hepatocytes and subcellular intestinal and hepatic tissue fractions. Among 12 quantified AKRs, the abundance of AKR1C4 and AKR7A3 increased with age, showing 10- and 9-fold higher abundance in adults than neonates, respectively. Similarly, out of 14 HSDs, nine isoforms showed age-dependent variability. HSD17β2 abundance decreased with age, which was twice as abundant in neonates compared to children (6-12 years), whereas the abundance of HSD17β6 and HSD17β13 increased with age, exhibiting 7- and 28-fold higher abundance in adults relative to neonates, respectively. HSD3β7, HSD17β2, HSD17β8, and HSD17β11 were more abundant in males, whereas AKRs, except AKR1A1, showed no significant sex differences. Subcellular localization analysis revealed that AKRs are predominantly localized in the cytosol, whereas HSDs are enriched in microsomes. AKR1B10, AKR7A3, HSD17β2, and HSD17β11 were primarily expressed in the intestine, whereas AKR1C2 and HSD17β10 were abundant in the liver. The conversion of testosterone to androstenedione exhibited strong correlations between protein abundance and activity for HSD17β2, HSD17β11, and HSD17β12. Significant protein-protein correlations were observed between HSD17β2 and HSD17β11 (r = 0.81) and AKR1A1 and AKR1E2 (r = 0.91). These findings provide valuable insights into physiological levels including the variability in AKRs and HSDs, which can support the development of physiologically-based pharmacokinetic models to predict the pharmacokinetics of their substrate drugs.