Comparison of three colorimetric reagents in the determination of methanol with alcohol oxidase. Application to the assay of pectin methylesterase

Three colorimetric reagents for the determination of formaldehyde, the Nash reagent (ammonia plus acetylacetone), Purpald (4-amino-3-hydrazino-5-mercapto-1,2,4-triazole), and N-methylbenzothiazolinone-2-hydrazone (MBTH), were compared for the determination of methanol when used in conjunction with alcohol oxidase. The combination of alcohol oxidase plus the commonly used Nash reagent was specific for methanol versus ethanol, but had the lowest sensitivity of the three reagents tested. Substituting Purpald for the Nash reagent increased the sensitivity 3-fold while still maintaining a high (59-fold) selectivity for methanol versus ethanol. Using MBTH increased the sensitivity still further, but with a loss of the selectivity toward methanol. Since MBTH reacted with aldehydes under neutral conditions, it could be included along with the alcohol oxidase to act as an aldehyde trap. This prevented further oxidation reactions by alcohol oxidase and allowed for extended incubations. A procedure for assaying low levels of pectin methylesterase activity that relies on this trapping ability is described. In addition, alcohol oxidase plus Purpald is shown to be a simple and sensitive way to measure the methanol released from plant material following the thermal activation of endogenous pectin methylesterase.