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Summary Holo and apoenzyme of aspartate aminotransferase from beef kidney are 80% inactivated by photoxidation in the presence of 2 × 10-6m tetraiodofluroescein with the modification of two histidine residues per enzyme protomer. At a higher concentration (1 × 10-5m) a tyrosine residue is also modified. The keto substrates, ketoglutarate and oxalacetate, protect the enzyme from photoxidation.

Summary Cytoplasmic aspartate aminotransferase from beef kindney loses 25% of its activity on nitration with tetranitromethane while the apoenzyme about 95%. In the holoenzyme 0.5 tyrosine residue and 1.0 tyrosine residue in the apoenzyme are nitrated per enzyme protomer. In addition 1 cysteine residue per protomer is oxidized in both. The presence of substrates,a-ketoglutarate and glutamate, both at ten times their Km values, does not change these results. Mercaptoethanol does not affect the residual activity of either the nitrated holo or apoenzyme. Dithionite abolishes the activity of the nitrated holoenzyme by reducing the coenzyme moiety. It has no effect on the native holoenzyme or on either the native or nitroapoenzyme.