A central cleavage at 15, 15' position of βーcarotene has not been an established theory in absorption and metabolism, because an exenteric cleavage of βーcarotene for producing apocarotenals has been suggested. We considered that this problem was based only on the imperfection of the assay method for βーcarotene-15,15'-dioxygenase (BCDC) activity in vitro, and tried the establishment of this assay method by investigating cofactor activities together with other assy conditions. BCDO activity of rat intestina mucosa homogenate could not be detected with the known method by Goodman et al. SH reagents (GSH or DTT) and nicotinamide (NA) were essential for BCDO assay. NADH could take place with NA, but the product was retinol instead of retinal. NAD+ partially inhibited the enzyme activity. Optimum concentrations of other cofactors were decided under the following conditions : 1 mM GSH, 1 mM Fe2+, and 10 mM glycocholate. From these results, a good reproducibility of the in vitro assay for BCDO activity was obtained, and it was confirmed that the central cleavage theory presented by Goodman was reasonable.
βーcarotene cleavage enzyme
rat intestine mucosa