Answer:
It is usually easier to calculate an enzyme's reaction velocity from the rate of appearance of PRODUCT rather than the rate of disappearance of a SUBSTRATE. Enzyme activity is measured as an INITIAL reaction velocity, the velocity before much SUBSTRATE has been depleted and before much PRODUCT has been generated. It is easier to measure the appearance of a small amount of PRODUCT from a baseline of zero PRODUCT than to measure the disappearance of small amount of SUBSTRATE against a background of high concentration of SUBSTRATE.
