CHEMICAL STUDY OF THE TOPOGRAPHY OF PORCINE LUTROPIN (LH)* USING DINITROFLUOROBENZENE AND DANSYL CHLORIDE

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Abstract
Treatment of porcine lutropin [luteinizing hormone, LH] .beta.-subunit by increasing amounts of dansyl chloride shows that only 1 fluorescent group can be bound to the free subunit, i.e., on tyrosine .beta.-37. This modification prevents reassociation with native .alpha.-subunit. In contrast to dansyl chloride, dinitrofluorobenzene reacts preferentially with tyrosine .beta.-59. This substitution does not interfere with the reassociation with the native .alpha.-subunit. By using equimolar ratio of dansyl chloride and porcine lutropin .alpha.-subunit it is possible to modify this subunit on a single site, which is found to be a tyrosine residue. The monodansylated .alpha.-subunit is still able to recombine with native .beta.-subunit, but its fluorescence is markedly quenched upon binding. The O-dansyl-tyrosyl fluorescence quenching by KI is more effective on the recombined dimer than on the free .alpha.-subunit. Both reconstituted dimers (native .alpha. .times. dinitrophenylated .beta. and dansylated .alpha. .times. native .beta.) are without biological activity [in the ovarian ascorbic acid test]. It is not clear whether the substituted phenolic groups are essential, or whether the added groups prevent the .alpha..beta. dimer from taking the active conformation. This alternative is discussed in the light of recent data from the authors'' laboratory and others.