Characteristics of covalent gossypol binding to microsomal proteins

Gossypol is a potent antifertility agent contained in seeds and other parts of cotton plants. The limit set in 1974 by the FDA for this C30H30O8 compound in consumer products is 450 ppm. The binding characteristics and the nature of the microsomal protein adducts of radiolabeled gossypol were studied using centrifugation, extraction, reverse phase HPLC and filter assay approaches. Results showed a significant amount of radiolabeled gossypol to be associated with the precipitated proteins after aqueous, ethanol, acetone and ether extractions. The nature of binding of these protein adducts involved covalent, covalent but reversible (e.g., Schiff bases), and tightly-bound and trapped noncovalent residues. Non-acid labile binding adducts constituted 40% of the precipitated microsomal proteins. Eight percent of the adducts were covalent, reversible and reducible by NaBH4. A gradient HPLC separation of the acetone extracts resulted in non-gossypolone hydrophilic protein adducts with a mean retention time of 2.3 minutes. Gossypol can bind tightly to hepatic microsomal proteins with a ratio of 80 nmoles/mg protein under physiological conditions. Significant portions of these bindings are not due to simple acid labile Schiff base formation. Purer membrane preparation provided results showing predominant binding of gossypol to endoplasmic reticulum (ER) and mitochondria, followed to a lesser extent by peroxisomes and plasma membranes. Difference spectra of the gossypol-bound rat hepatic microsomal preparations and controls demonstrated a 3 nm shift from 413 to 410 nm caused by gossypol covalent-binding. Results of this study indicate that gossypol binds covalently to microsomal proteins. Its binding to membrane proteins may affect metabolism of sterols, steroids, or fatty acids.