Sites of direct and indirect halogenation of albumin

The sites of radiohalogenation in proteins vary with the labeling method and the pH of the labeling reaciton. We have directly halogenated albumin with carrier-free radioiodide by three methods (pH range 2.2--9.3), and with carrier-free radiobromide by the chloroperoxidase method (pH range 2.2--4.6). Albumin was also indirectly halogenated by attaching a radioiodinated acylating agent, N-succinimidyl-3-(4-hydroxyphenyl) propionate (SHPP). The labeled proteins were proteolyzed enzymatically at neutral pH and the labeled amino acids produced were analyzed by liquid chromatography. Iodination at pH 7 yielded predominantly monoiodotyrosine, but at lower pH, fewer tyrosyl residues are labeled and a greater number of unstable sulfur-iodine bonds are formed at cysteinyl residues. Bromination with chloroperoxidase resulted in a high degree of labeling of cysteinyl residues at pH 2.8, the condition for optimum activity of this halogenating enzyme. Indirect halogenation with SHPP resulted in labeling of mid-chain lysyl, histidyl and tyrosyl residues.