Purification and characterization of aromatase from human placenta.

Aromatase from human placenta has been purified to homogeneity (MW 55,000). Enzymatic activity can be reconstituted with reductase from pig liver in an aqueous buffer or after incorporation of the enzyme into liposomes. In both cases the enzyme converts androstenedione to estrone and testosterone to estradiol. Aromatase shows a typical CO-spectrum when reduced with dithionite and a type I spectral shift with both substrates. The NH2 terminal amino acid sequence is hydrophobic but shows no homology to that of other cytochromes P-450. Five cysteine peptides have been isolated by HPLC following tryptic digestion of the [14C]-carboxymethylated protein. Amino acid sequences of these peptides reveal that histidine is the carboxy-terminal amino acid of the protein and that significant homology exists with corresponding peptides from other cytochromes P-450. Unique oligonucleotides (62 and 30 MER) synthesized on the basis of a 45 amino acid sequence near the center of the molecular have been used to clone the aromatase gene from a cDNA expression library from human placenta in lambda gt11.