Catalyzes the C6-oxidation step in brassinosteroids biosynthesis (PubMed:15710611). Converts 6-deoxocastasterone (6-deoxoCS) to castasterone (CS), and castasterone (CS) to brassinolide (BL) (PubMed:15710611). 6-deoxocastasterone + O2 + reduced [NADPH--hemoprotein reductase] = 6alpha-hydroxycastasterone + H(+) + H2O + oxidized [NADPH--hemoprotein reductase] 6alpha-hydroxycastasterone + O2 + reduced [NADPH--hemoprotein reductase] = castasterone + H(+) + 2 H2O + oxidized [NADPH--hemoprotein reductase] castasterone + O2 + reduced [NADPH--hemoprotein reductase] = brassinolide + H(+) + H2O + oxidized [NADPH--hemoprotein reductase] 6-deoxocastasterone + 2 O2 + 2 reduced [NADPH--hemoprotein reductase] = castasterone + 2 H(+) + 3 H2O + 2 oxidized [NADPH--hemoprotein reductase] Plant hormone biosynthesis; brassinosteroid biosynthesis. Expressed in fruits. Belongs to the cytochrome P450 family.