A total of 40 methanol extracts (20 μg/ml) from Thai vegetables and fruits, particularly those used as flavours or condiments, were submitted to the EBV activation test, in which teleocidin B-4 (20 ng/ml), a potent TPA-type tumour promoter52, and sodium n-butyrate (3mM), were used as the EBV activator48. EBV genome activation was measured by the level of induction of viral early antigen (EA). Possible anti-tumour promoting activity was determined by the ratio of EA-induced cells treated with a test sample as compared with that tested with only n-butyrate and teleocidin B-4. The inhibitory effect (IE) of each test extract was classified as follows: +++ strongly active (IE ≥ 70%); ++ moderately active (70% > IE ≥ 50%); + weakly active (50% > EI ≥ 30%); - inactive (30% > IE). In addition, cell viability (CV) was also classified as follows: ++ highly toxic (30% ≥ CV); + moderately toxic 70% ≥ CV > 30%; - non-toxic (CV > 70%).
Screening data (Table 2) shows that 31 species, consisting of 14 strongly, nine moderately, and eight weakly active species, exhibited significant inhibitory activity toward EBV activation. As shown in Figure 1, the proportion of strongly active plants (35 % of the total) to the whole was about three times higher than that (12%) in the screening test of Japanese common edible plants tested previously41, though there is an obvious difference in their sample numbers. The experimental conditions in the present test seem 1000 to be stricter than those in the previous screening test for Japanese common edible plants, since the relative concentrations of the extracts to the tumour promoter were five times less in the present assay than in the previous test41. It is, accordingly, indicated that our criteria in plant selection centring on flavours, condiments etc could greatly facilitate the search for natural sources of potent anti-tumour promoters. One might ask if there are notable differences in activity between the common nutritive Thai and Japanese plants. To clarify this, further screening tests of 150 common edible plants from Thailand are now being undertaken.