The Tricarboxylic Cycle and Respiration

Glycolysis and the tricarboxylic cycle are fundamental to respiration, that is, the uptake and utilization of oxygen from the air. If respiration is shut down, suffocation results. It is the way in which many poisons act, for instance, the deadly South 

American poisonous mixture called curare. Curare is derived from certain members of the genus Strychnos, and contains a number of toxic alkaloids (strychnine being a famihar name for such an alkaloid). Thus, enzymes in glycolysis and the tricarboxylic cycle proper are of fundamental concern, for their blockage or inhibition could prove fatal. Inhibitors for enzymes involved in the carboxylic acid cycle are listed in Appendix B of Hoffman (1999). 

Both enzymes mentioned previously pertain to the tricarboxylic acid cycle proper, and are apparently vital to sustaining respiration. Accordingly, inhibitors can lead to undesirable and sometimes life-threatening consequences. Enzyme inhibitors for pyruvate dehydrogenase are more commonly known, those for a-ketoglutarate dehydrogenase less so. In the Jain reference respiration inhibitors are treated as a special category. 

Among the other notations is that acetaldehyde is to be avoided, it being a respiration inhibitor. It is related to ethyl alcohol or ethanol and also to acetic acid, but not necessarily to citric acid as involved in the carboxylic acid or citric acid cycle. Alkaloids are expectedly respiration inhibitors, and anesthetics can have respiration inhibition as a side effect. Aromatic adds such as phenol are bad news, as are arsenate, cyanide, isothiocyanate, and thiocyanate. The heavy metals cadmium, cobalt, copper, ruthenium, vanadate, and zinc are regarded as health risks, if not for respiration, for other reasons. 

The inclusion of fatty acids is a surprise, but maybe not for that called guaiaretic acid, nordihydro, better known as NDGA or nordihydroguaiaretic acid. It is an ingredient in chaparral or creosote bush, and was formerly used commercially as an oxidation inhibitor in various apphcations (note that oxidation inhibitors are viewed as beneficial for a number of purposes, including cancer suppression). 

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