Binding of the chymotrypsin substrate, tryptophan methyl ester, by rat alpha-fetoprotein

Conformational changes in rodent and human alpha-fetoprotein: influence of fatty acids

Binding, spectral and immunological studies were performed to demonstrate the conformational changes in rodent and human alpha-fetoprotein (AFP) induced by a free fatty acid environment. Scatchard analysis of estradiol (E2) binding to purified rat AFP indicated that unsaturated fatty acids changed the number of binding E2 sites and the apparent E2 equilibrium dissociation constant which varied non-linearly with docosahexaenoic acid concentration. UV spectral analysis of rodent and human AFPs showed that the absorbance minimum of AFP incubated with unsaturated fatty acid (L-AFP) was red-shifted, broadened and less pronounced than that of purified native AFP (N-AFP). Immunochemical studies with specific polyclonal antibodies to purified rodent and human AFPs (N-AFP antibodies) showed that these proteins lost immunoreactivity after incubation with unsaturated fatty acid. N-AFP antibodies recognized fewer epitopes on L-AFP than on N-AFP, whatever the species. Specific anti-rat L-AFP antibodies were used to demonstrate specific epitopes on rat L-AFP. Rat L-AFP antibodies did not recognize rat N-AFP. Saturated fatty acids were without effect on the binding, spectral and immunological properties of rodent and human AFPs. RIA or ELISA values for human AFP from fetal serum, hepatoma serum, and cord serum, were reduced 80, 50 and 5%, respectively, by unsaturated fatty acids. This decrease correlated with the relative percentage of polyunsaturated fatty acid in each biological fluid. Such results indicate that an unsaturated fatty acid environment induces conformational changes in AFP which may modulate the endocrine and immune functions of this protein.

Gossypol inhibits estrogen binding to rat alpha-fetoprotein

We find that gossypol, a male anti-fertility compound, is a reversible competitive inhibitor of estrogen binding to rat alpha-fetoprotein (AFP). The Kd of gossypol for rat AFP is 1.75 microM, which is similar to gossypol's affinity for lactate dehydrogenase isozyme X, the putative site where gossypol exerts its anti-fertility effects. Reacting sodium cyanoborohydride with gossypol reduces its affinity for AFP, showing that intact aldehyde groups on gossypol are important for binding to rat AFP and indicating that gossypol is specifically inter-acting with a nucleophilic site on AFP that influences estrogen binding.

Chick oviduct progesterone receptor binding of 15 beta, 17-dihydroxyprogesterone and its analogues

We have tested derivatives of progesterone obtained by fermentation with Aspergillus giganteus for relative binding affinity for the progesterone receptor of chick oviduct. Our studies show that hydroxyl and oxo substitutents at C-11 and C-15 of progesterone significantly decrease the hormone's affinity for the progesterone receptor. The loss in affinity on introduction of a C-15 hydroxyl in 17-hydroxyprogesterone is restored by acetylation to 15 beta-acetoxy-17 hydroxyprogesterone. The latter compound may have some potential as an in vivo agent.

Inhibition of estrogen binding to rat alpha-fetoprotein by tryptophan p-nitrophenyl esters

Rat alpha-fetoprotein (AFP) contains a site that both binds the protease substrate tryptophan methyl ester (TrpOMe) and influences estrogen binding. We have studied the effect of changing the amino acid and ester portions of this compound on its binding to AFP, as measured by the ability of the ester to inhibit binding of [3H]-estrone to AFP. We find that: (1) AFP binds tryptophan esters better than phenylalanine or tyrosine esters, (2) substitution of butyl or benzyl for the methyl group in TrpOMe increases binding for AFP by 30- to 100-fold, (3) substitution of p-nitrophenol at the ester position increases binding for AFP by 10(5), (4) p-nitrophenyl substitution in the ester position of tyrosine or phenylalanine methyl ester increases the affinity for AFP by 10(3) to 10(4), (5) inhibition of estrogen binding to AFP by tryptophan p-nitrophenyl esters is reversible and competitive, and (6) the hydrolysis products of tryptophan p-nitrophenyl ester are ineffective in inhibiting estrogen binding to AFP. Based on these results, we suggest that AFP contains a tryptophan ester recognition site which binds p-nitrophenyl esters with high affinity and influences estrogen binding. The ester binding site may be located spatially near the estrogen binding site.