Tenace sites

Mora, M.A., H.J. Auman, J.P. Ludwig, J.P. Giesy, D.A. Verbrugge, and M.E. Ludwig. 1993. Polychlorinated biphenyls and chlorinated insecticides in plasma of Caspian terns relationships with age, productivity, and colony site tenacity in the Great Lakes. Arch. Environ. Contam. Toxicol. 24 320-331. 

Different enzymes and different tissue sites differ in the tenacity with which they can retain flavin coenzymes in times of riboflavin deficiency, so there is a characteristic pecking order for flavoen-zyme protection, which appears to reflect the metabolic importance of the different metabolic pathways affected. Apart from this pecking order, however, there is no repository of unused or nonfunctional riboflavin that can act as a store in times of dietary deficiency. Although some organs (such as liver) have relatively high concentrations of flavin enzymes, all of the flavin seems to be present as coenzyme moieties of flavin holoenzymes. Each tissue has a characteristic ceiling level of riboflavin at saturation, and a floor level characteristic of severe depletion, and these are determined, respectively, by the total amount of apoflavoprotein, and the amount of resistant holoenzyme, which cannot be depleted of its cofactor during riboflavin deficiency. 

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