Aromatization

Aromatization is the reaction catalysed by aromatase, the product of the single gene CYP19A1. It converts the saturated A-ring of a 19-carbon androgen into the phenolic A-ring of an 18-carbon estrogen through three oxygen- and NADPH-dependent hydroxylations, expelling carbon 19 as formic acid. Testosterone becomes estradiol, androstenedione estrone; the enzyme redirects androgen rather than adding to it.

Aromatase is expressed in ovarian granulosa cells, the placenta, bone, brain and, in men, adipose stromal tissue, where most male circulating estradiol is made rather than in the testis. Anastrozole and letrozole inhibit it competitively and reversibly, exemestane is a steroidal analogue that inactivates it irreversibly, and all three are approved for hormone-receptor-positive breast cancer, with letrozole also used for ovulation induction. Rare loss-of-function mutations in CYP19A1 produce men with high testosterone whose growth plates never close and whose bone density falls, the natural experiment establishing estradiol rather than testosterone as the hormone that closes epiphyses.

Because the substrate is androgen, anything that raises testosterone raises estradiol, and more adipose tissue converts more of it. That is the tradeoff behind suppressing the enzyme: estradiol in men carries bone density, lipid and libido effects, so driving it low is not a neutral act of tidying a lab panel.

The commonest error treats estradiol in men as a pure side effect with an optimal value of zero; the second is measuring it badly while doing so. Immunoassay estradiol is unreliable at concentrations typical of men, so the number acted on may not separate a real elevation from assay noise. Over-the-counter aromatase inhibitor supplements compound both, since a potent enzyme-inhibition claim describes a drug effect the product has never been tested for.