The low temperature spectra of hemoproteins. II. Cytochromes of heart muscle preparations.

The classical work of Keilin and Hartree (1)) based primarily on microspectroscopic and enzymatic studies, has served as a foundation for our present understanding of terminal electron transport. The application of spectrophotometric techniques was generally limited to studies of soluble purified pigments such as cytochrome c, or of preparations treated with clarifying agents such as sodium cholate, until the development of rapid and sensitive spectrophotometric methods by Chance (2). By means of such techniques, which permit the measurement of small spectral changes in turbid solutions, Chance has been able to deal quantitatively with the functional relationships of the respiratory pigments. Recently the wave length-scanning spectrophotometer, devised by Chance and his coworkers (3-6), has been modified to permit the recording of spectra of samples cooled to low temperature (7), simulating the technique first applied by Keilin and Hartree to the microspectroscope (8). Spectrophotometric studies at low temperatures make it possible to identify clearly those pigments of the respiratory chain which at room temperature are indistinguishable one from another. This is best exemplified by the resolution of the absorption bands of cytochromes c, cl, and b. In addition, the visible absorption bands of hemoproteins are greatly intensified at low temperatures, permitting measurements of pigments which are of such low concentration that they cannot be determined by measurement at room temperature. In addition to the recent advances in spectrophotometric techniques, the electron transfer systems have been isolated in highly active form. A particulate enzyme (ETP) isolated by Green et al. (9) catalyzes the oxidation of succinate and reduced diphosphopyridine nucleotide (DPNH) by molecular oxygen without the addition of external cytochrome c. DPNH oxidase, a derivative form of ETP without succinoxidase activity, has been