Catalyzes the NAD(P)(+)-dependent oxidative decarboxylation of the C4 methyl groups of 4-alpha-carboxysterols in post-squalene cholesterol biosynthesis (By similarity). Also plays a role in the regulation of the endocytic trafficking of EGFR (By similarity). 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. Brain, heart, liver, lung, kidney, skin and placenta. The disease is caused by variants affecting the gene represented in this entry. The disease is caused by variants affecting the gene represented in this entry. Belongs to the 3-beta-HSD family.