Limited Activity of Carotene Dioxygenase

Other carotenoids in the diet, which are not substrates, such as canthaxan-thin and zeaxanthin, may inhibit carotene dioxygenase and reduce the proportion that is converted to retinol (Grolier et al., 1997). Similarly, a variety of antioxidants that occur in foods together with carotenoids, including flavo-noids (Section 14.7.2), also inhibit carotene dioxygenase. 

A number of studies have reported low semm concentrations of retinol and high concentrations of /3-carotene in patients with insulin-dependent diabetes mellitus. Krill and coworkers (1997) showed that up to one-third of nondiabetic first-degree relatives of patients with diabetes also showed a low serum retinokcarotene ratio, implying a genetic predisposition to low activity of carotene dioxygenase, possibly associated with insulin-dependent diabetes. 

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