There are also many plant carotenoids, some of which, such as (3-carotene and (3-cryptoxanthin (figure 1) can be converted to vitamin A by an oxygenase present in the intestine and elsewhere. Rich sources of provitamin A carotenoids include green leaves and most yellow and orange fruits and vegetables, and it has been estimated that between one-twelfth and one-half by weight of such carotenoids can be converted to vitamin A (the efficiency of conversion being very variable, and dependent on many different factors such as dietary fat content, accessibility of dietary carotenoids to absorptive surfaces and enzymes, and bile salt concentrations). Some carotenoids cannot be converted to vitamin A, but even those that are not (such as lutein) may still possess antioxidant properties which make them potentially important nutrients. Carotenoids are absorbed in the ileum and are detectable in various tissues, but unlike vitamin A they do not combine with specific transport proteins and are transported mainly as non-polar lipids.