To measure an enzyme is in some ways easier, but in many ways harder than to measure a metabolite. It is easier because an enzyme is a catalyst and can usually be made to generate a great deal more of its product than is present in the tissue of origin. For example, muscle lactate dehydrogenase can generate in an hour 100,000 times more lactate than is ever present in resting muscle. It is harder to measure an enzyme because, unlike metabolites, which always have identical properties wherever they are found in the biological world, enzymes having the same function can differ greatly. Each enzyme that catalyzes a particular reaction will more often than not differ from species to species, from cell type to cell type, and even from one kind of organelle to another. The differences may concern kinetic properties, optimum pH, sensitivity to activators and inhibitors, and so forth. In addition, the activity of every enzyme is affected by temperature, pH, and usually by ionic strength, type of buffer, and so forth. Finally, an enzyme is in a real sense alive, and most enzymes can easily die or be killed. All of this presents a challenge to the analyst, especially when the protocol has not been designed for the particular cell type under examination.