132 BARRETI & KWAN the waste products of protein metabolism; he reasoned that their tissues might contain breakdown products that, in other experimental animals, were too rapidly eliminated for detection. He began a search for such elusive nitrogenous waste products in dogfish, which led him to the fortuitous discovery of trimethylamine oxide (TMAO)(1 1 5). Later Suwa hypothesized that trimethyl amine (TMA), already known to be a major contributor to the odor of spoiling fish, was the result of the reduction ofTMAO by contaminating bacteria (11 6). Experimental support for the hypothesis did not appear until 1937, when Beatty & Gibbons (12) demonstrated the relationship between TMA production and increases in bacterial counts during the storage of marine fish. They suggested that assays ofTMA might be a good way to evaluate fish freshness, and their report precipitated an intense interest in bacterial TMAO reduction by scientists associated with fisheries. Beatty (10) showed that the appearance of TMA was in fact correlated with the disappearance of TMAO and suggested that the reduction ofTMAO might constitute some type of bacterial respiration. Watson (123, 124) demonstrated that TMAO facilitated the anaerobic growth of certain bacterial isolates from spoiling fish but was not reduced by these bacteria growing in the air. These observations were soon followed by pre liminary studies of the" triamineoxidease"(119, 120) with respect to its occurrence in bacteria (3, 26, 1 19, 123-125), its specificity (120), the glycolyt ic products associated with its reduction (34), and its sensitivity to inhibitors (24, 28, 42, 76). Neilands (76) suggested that reduction ofTMAO involved an electron transport system. Unfortunately these studies could not provide a strong foundation for subsequent studies of TMAO reduction because the bacterial cultures were neither well characterized nor preserved, and we now know that TMAO reduction occurs in a number of unrelated bacteria. The suggested importance of TMAO reduction to the concerns of marine fish storage was supported by a number of surveys of TMAO content in fish tissue that showed that the concentration of this compound in all marine fish and molluscs examined was indeed very high, frequently more than 1% of the wet weight (11, 44, 51, 79, 98). On the other hand its concentration in freshwater fish was shown to be negligible (1 1, 44, 98). The most extensive survey of TMAO content in marine organisms was that of Groninger (51). The origin of TMAO in fish has not been definitely established, but there is evidence that it is either synthesized from TMA, which is found in marine algae (111), or absorbed as TMAO from fish lower on the food chain (13). It has been speculated that TMAO is important to the osmotic balance in the tissues of marine fish (see 14 and 51 for reviews). Interest in bacterial TMAO reduction began to wane when it became clear that TMA was not the only major indicator of spoilage and that the correlation between TMA concentration and bacterial growth or between TMA concentra tion and stage of spoilage was, at best, very rough (see 1 21 for review). In the