BIOLOGICAL CHEMILUMINESCENCE

Low-level chemiluminescence by living systems originates from the biochemical generation of electronically excited states. The excited singlet and/or triplet species may be formed in the cellular milieu in high yields, but unless they have a chance to radiate they will go undetected; indeed these excited states may be almost totally quenched by undergoing cellular physical and chemical processes. For example, several aminoacids (present within cells in mM range) and proteins (Matheson et a l . , 1975), vitamin C (Rooney, 1983; Chou and Khan, 1983), vitamin E (Grams and Eskins, 1972; Foote, 1976), vitamin A (Smith, 1983), NADPH, NADH, and certain components of the mitochondria1 electron transport chain (Bodaness, 1982), among others, are effective quenchers of singlet oxygen. This shows that cells are endowed with a variety of physical and chemical quenchers of excited species, making spontaneous chemiluminescence of living tissues (Barenboim et al., 1969; Cadenas et al . , 1984a) hardly detectable. Although detection and identification of these excited states is rather easily achievable in well-defined chemical systems (or biologically-oriented model systems), however, the task becomes much more problematical with increasing complexity of the biological system under study, e.g., in the case studies of isolated cells 3r organs. Moreover, photoemission from complex living systems exhibits an intricate spectral distribution, thus revealing that light emission might be more complex than that observed in the relaxation of a sole excited state to the ground state. Two feasible routes can be followed for the investigation of low-level chemiluminescence. One is to exhaust the endogenous quenchers; the other is to reduce the complexity of the intact system by subfraction or by examining enzymatic model systems. There are multiple reports on the generation of electronically excited states during metabolic reactions, the study of which was carried out under well-defined and controlled experimental conditions. These chemiluminescent reactions will also occur within the cellular environment, where, however, it would be more difficult to detect a significant chemiluminescence signal. This review covers those recent biological chemiluminescence systems in which evidence has been 003 1-8655184 $03 .00+0.00 Copyright@ 1984Pcrgamon PrescLtd