The final step in hydrocarbon biosynthesis involves loss of CO from a fatty aldehyde. This decarbonylation is catalyzed by microsomes from Botyrococcus braunii. Among the several detergents tested for solubilizing the decarbonylase, octyl beta-glucoside (0.1%) was found to be the most effective and released 65% of the enzyme activity in soluble form. FPLC of the solubilized enzyme preparation with Superose 6 followed by ion-exchange FPLC with Mono Q resulted in 200-fold increase in specific activity with 7% recovery. The purified enzyme released nearly 1 mol of CO for each mol of hydrocarbon. SDS/PAGE of the enzyme preparation showed two protein bands of equal intensity at 66 and 55 kDa. The absorption spectrum of the enzyme with bands at 410 nm, 425 nm, 580 nm, and 620 nm suggests the presence of a porphyrin. Electron microprobe analysis revealed that the enzyme contained Co. Purification of the decarbonylase from B. braunii grown in 57CoCl2 showed that 57Co coeluted with the decarbonylase. These results suggest that the enzyme contains Co that might be part of a Co-porphyrin, although a corrin structure cannot be ruled out. Co-protoporphyrin IX itself caused decarbonylation of octadecanal at 60 degrees C, whereas the metal ion or protoporphyrin alone, or several other metal porphyrins, did not cause decarbonylation. These results strongly suggest that biosynthesis of hydrocarbons is effected by a microsomal Co-porphyrin-containing enzyme that catalyzes decarbonylation of aldehydes and, thus, reveal a biological function for Co in plants.