Catalyzes the NAD(P)(+)-dependent oxidative decarboxylation of the C4 methyl groups of 4-alpha-carboxysterols in post-squalene cholesterol biosynthesis (PubMed:10369263, PubMed:14567972). Plays a role in the regulation of the endocytic trafficking of EGFR (PubMed:23125191). a 3beta-hydroxysteroid-4alpha-carboxylate + NADP(+) = a 3-oxosteroid + CO2 + NADPH a 3beta-hydroxysteroid-4alpha-carboxylate + NAD(+) = a 3-oxosteroid + CO2 + NADH 4alpha-carboxyzymosterol + NADP(+) = CO2 + NADPH + zymosterone 4alpha-carboxy-4beta-methyl-5alpha-cholest-8-en-3beta-ol + NADP(+) = 4alpha-methyl-5alpha-cholest-8-en-3-one + CO2 + NADPH 4alpha-carboxy-5alpha-cholest-8-ene-3beta-ol + NADP(+) = 5alpha-cholest-8-en-3-one + CO2 + NADPH 4beta-methylzymosterol-4alpha-carboxylate + NADP(+) = 3-dehydro-4-methylzymosterol + CO2 + NADPH 4beta-methylzymosterol-4alpha-carboxylate + NAD(+) = 3-dehydro-4-methylzymosterol + CO2 + NADH 4alpha-carboxy-5alpha-cholest-8-ene-3beta-ol + NAD(+) = 5alpha-cholest-8-en-3-one + CO2 + NADH 4alpha-carboxy-4beta-methyl-5alpha-cholest-8-en-3beta-ol + NAD(+) = 4alpha-methyl-5alpha-cholest-8-en-3-one + CO2 + NADH 4alpha-carboxyzymosterol + NAD(+) = CO2 + NADH + zymosterone Steroid biosynthesis; zymosterol biosynthesis; zymosterol from lanosterol: step 4/6. Homodimer. Trafficking through the Golgi is necessary for ER membrane localization. Defects in Nsdhl are the cause of male-lethal mutations bare patches (Bpa) and striated (Str) phenotypes. Heterozygous Bpa females are dwarfed and demonstrate abnormal deposits of calcium in tail vertebrae. They also develop a hyperkeratotic skin eruption shortly after birth that resolves, leaving 'bare patches' arranged in a horizontal, striped pattern. The Str females are indistinguishable from normal littermates until postnatal day 12-14 when they develop striations in their coat. Belongs to the 3-beta-HSD family.