| Abstract: | 3H]Nitrendipine and high intensity ultraviolet irradiation have been used to photoaffinity label the protein component of the high affinity nitrendipine-binding site in subcellular membrane fractions from canine cardiac muscle. Irradiation of isolated cardiac membranes in the presence of 3H]nitrendipine resulted in the covalent labeling of a protein component that migrated on sodium dodecyl sulfate-polyacrylamide gels with an apparent molecular weight of 32,000. Incorporation of 3H]nitrendipine did not occur in the absence of irradiation. The photoaffinity labeling of the 32,000-Da protein by 3H]nitrendipine was inhibited by excess unlabeled nitrendipine, nifedipine, or verapamil. EDTA, ATP, and La3+, which are known to reduce high affinity nitrendipine binding, also inhibited the photoaffinity labeling of this membrane protein by 3H]nitrendipine. The 32,000-Da 3H]nitrendipine-labeled protein was found to be enriched in the ryanodine-sensitive fraction of cardiac sarcoplasmic reticulum and absent from the ryanodine-insensitive fraction of cardiac sarcoplasmic reticulum which is known to lack high affinity nitrendipine binding. Therefore, the 32,000-Da photoaffinity-labeled 3H]nitrendipine-binding protein exhibits properties identical to those expected for the protein component of the high affinity nitrendipine-binding site in isolated cardiac membranes. |
