Inactivation of Pepsin by Iodine

1. Pepsin solutions were iodinated at pH 5.0-6.0 until 10-20 per cent of the activity was lost and 1/20 (0.7 per cent) of the saturating amount of iodine had been introduced into the protein molecule. After alkaline hydrolysis 65 per cent of the original iodine was accounted for as mono-iodotyrosine although only 42 per cent was isolated as a crystalline product. No evidence was obtained to support the possibility that any group other than tyrosine in pepsin was iodinated. 2. Some of the properties of the crystalline l-mono-iodotyrosine were determined and compared to those of di-iodotyrosine. 3. One iodinated pepsin preparation was crystallized. The crystal form was the same as that of the original pepsin. A solubility curve of the crystals demonstrated that it was very different from pepsin and had nearly constant solubility.