Starch-iodine assay method underestimates alpha-amylase inhibitory potential of antioxidative compounds and extracts

Starch-iodine assay method for the determination of α-amylase activity is also used in screening extracts for α-amylase inhibitors. However, there are indications that this method may not be appropriate for screening some classes of compounds or plant extracts. The present study investigated the limitation(s) of this method in screening plant extracts/compounds for α-amylase inhibition. A crude methanol extract (CR) of Dacryodes edulis, its solvent fractions (ethyl acetate (EA), aqueous methanol (AM), and hexane (HX)), quercetin (QC), and benzoic acid (BA) were used for this study. The phytochemical content and antioxidant activity were screened spectrophotometrically. α-Amylase inhibition (expressed in percentage and as IC50) was determined by starch-iodine method approach I and II (ST-ID I and ST-ID II, respectively) and dinitrosalicylic acid (DNSA) as the control method. The results showed that the extracts/compounds (AM, EA, and QC) with significantly high polyphenolic content, antioxidant activity, and starch-iodine complex decolorization effect yielded contrary results of α-amylase inhibition when the results of ST-ID I and II methods were compared to that of the DNSA method. The other test samples (CR, HX, and BA) yielded similar results for all the three methods. The result also showed the decolorization (%) of starch-iodine complex by the test samples correlated significantly (r = 0.877, P < 0.05) with DPPH reduction (%). In conclusion, the present study showed that the starch-iodine method is not appropriate for screening antioxidative extracts/compounds for α-amylase inhibitors – they decolorize the assay reagent in a manner similar to DPPH reduction and hence confound the result.