How are luciferin and luciferase used today

Luciferin is used, for example, in reporter gene assays to study gene regulation and function where expression of the luciferin-tagged reporter is a marker to indicate successful uptake of the gene of interest in recombinant DNA techniques. Luciferase is a heat-sensitive protein that is used in studies on protein denaturation, testing the protective capacities of heat shock proteins. Luciferase is an enzyme used for bioluminescence by various organisms in nature, most famously the firefly. The scientist produces a construct in which the regulatory region of a target gene is fused with the DNA coding sequence for luciferase

Explanation:

where in your body is there an enzyme that does function at a high or low ph? what is the name of that enzyme and its function?