Studies on the brush border membrane of mouse duodenum. III. Iodination of membrane proteins.

Microvillus membranes were iodinated from luminally administered lactoperoxidase, H2O2 and 125I before and after neuraminidase treatment. Membranes were isolated, solubilized in sodium dodecyl sulphate buffer and electrophoresed on gels. Gels were stained for protein, and then sliced for liquid scintillation counting. When membranes were not treated with neuraminidase, the nonpermeating iodination probe attached only to a band containing protein of 150,000 daltons approximate molecular weight. This size class of protein may reside on the luminal side of the brush border membrane as opposed to the serosol side. Qualifications of this statement are discussed with reference to the location of tyrosyl residues and to the possibilities of masking molecules. Membranes treated from the luminal side with neuraminidase to remove possibly masking carbohydrate and then iodinated, appeared to contain an additional protein of estimated molecular weight 220,000 daltons which was accessible to 125I. Thus, a 220,000 dalton protein may also be on the luminal side of the membrane. An explanation is attempted for the discrepancy between the very few proteins labelled and the many proteins involved in terminal digestion and transport which would all be expected to be available to luminally administered iodination probe. Membranes were isolated, exposed to iodination and then solubilized for electrophoresis. Nearly all proteins were labelled, which indicated that there is an asymmetric distribution of proteins in the plane of the membrane. The two smallest molecular weight polypeptides which were not iodinated were proposed to be so disposed in the membrane that they were inaccessible to the probe, from either side.