Synthesis of a spin-labeled photoaffinity ATP analogue,and its use to specifically photolabel myosin cross-bridges in skeletal muscle fibers

A spin-labeled photoaffinity ATP analogue 3(2)-O-{4-[4-oxo-(4-amido-2,2,6,6-tetramethyl-piperidino-1-oxyl)]-benzoyl}benzoyl adenosine 5-triphosphate (SL-Bz₂ATP) was synthesized and used to photolabel myosin in muscle fibers. Previous work has shown that 3(2)-O-(4-benzoyl)benzoyl adenosine 5-triphosphate (Bz₂ATP) photolabeled Ser-324 of the 50 kDa tryptic fragment of skeletal S1 heavy chain. In this work, [-³²P]SL-Bz₂ATP was hydrolyzed and trapped as the diphosphate analogue with Co²⁺ and orthovanadate at the active site of myosin in rabbit psoas muscle fibers. After UV irradiation, the myosin heavy chain was the only protein band found to be significantly photolabeled as assayed by gel electrophoresis and radioactivity counting. The labeling was localized after brief trypsin digestion by SDS-PAGE to be on the 50 kDa tryptic fragment of the S1 heavy chain. Ca. 35% of the myosin in fibers was covalently photolabeled. The fibers photolabeled with SL-Bz₂ATP had the same active tension and maximum shortening velocity as the control fibers. The resulting spin label on myosin was too mobile to report the orientation of the heads in fibers. Nonetheless, this is the first work to show the feasibility of utilizing active site binding and photoaffinity labeling to place covalent spectroscopic probes at the myosin active site in fibers with high specificity and yield without affecting mechanical function.