Spin labeling in EPR of wild-type GALDH and C340A. A, Spin-labeling reaction of wild-type GALDH with 3-maleimido-PROXYL via Cys-340. B, Normalized absorption spectra of wild-type GALDH (solid line) and C340A (dotted line) labeled with 3-maleimido-PROXYL recorded in 50 mm sodium phosphate, pH 7.4. C, EPR spectra of wild-type GALDH modified with the 3-maleimido-PROXYL spin label (I), GALDH C340A modified with the 3-maleimido-PROXYL spin label (II), and free spin label (III). A sample of 30 μm spin-labeled enzyme or free spin label in 50 mm sodium phosphate, pH 7.4, was used.