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Studies of glutamate dehydrogenase: chemical modification and quantitative determination of tryptophan residues.

The effect of the modification of beef liver glutamate dehydrogenase with 2-hydroxy-5-nitrobenzyl bromide was studied with respect to the association-dissociation equilibrium as well as the enzymatic and regulatory properties. Upon incorporation of 2.1 molecules of the reagent per polypeptide chain, the association of the enzyme is strongly reduced, whereas the enzymatic activity is only slightly decreased (80% maximum velocity). The Km values and the regulation by ADP and GTP, respectively, remain unaltered. Evidence is presented that only tryptophan residues are modified. Magnetic circular dichroism measurements of the modified enzyme suggest partial disubstitntion of tryptophan residues. From the reduced incorporation of 2-hydroxy-5-nitrobenzyl groups at high enzyme concentration it is concluded that tryptophan is located at the association areas of the enzyme. 
 
 
 
Quantitative determination of the tryptophan content of glutamate dehydrogenase with 2-hydroxy-5-nitrobenzyl bromide, magnetic circular dichroism and peptide analysis yields four tryptophan residues per polypeptide chain contrary to three residues previously suggested from structural analysis. The fourth tryptophan, not present in the reported sequence, is recovered in a chymotryptie peptide Glu-Trp.