Uncoupler resistance in Escherichia coli

possible inhibition of absorption by folic acid as it interfered with the PAB assay. We conclude that PAB passes across human buccal mucosa by a non-saturable, Na+ -independent process presumably physical diffusion. These transport characteristics of PAB are similar to those found with small intestine of rats and hamsters in vitro (Spencer et al., 1966; Arvanitakis, 1978). PAB will be 92% ionized at p H 6 as in our experiments. Presumably the un-ionized molecule will be the permeant species. So PAB is like some other vitamins which are absorbed in a similar way across the mucosa of both the human buccal cavity and mammalian small intestine. This similarity has been seen with ascorbic acid (Sadoogh-Abasian & Evered, 1979), nicotinic acid and nicotinamide (Sadoogh-Abasian & Evered, 1980; Evered et al., 1980), and thiamin (Evered & Mallett, 1983; Evered & Moor, 1985). Surprisingly perhaps, buccal mucosa is also permeable to another vitamin-like compound, choline, which is also highly ionized at physiological pH values. Absorption was also Na+-independent (D. F. Evered & F. Mynah, unpublished work).