Nitric oxide may itself regulate NOS expression and activity. Specifically, NO has been shown to play an important negative feedback regulatory role on NOS3, and therefore vascular endothelial cell function [ citation needed ] . This process, known formally as S -nitrosation (and referred to by many in the field as S -nitrosylation), has been shown to reversibly inhibit NOS3 activity in vascular endothelial cells. This process may be important because it is regulated by cellular redox conditions and may thereby provide a mechanism for the association between "oxidative stress" and endothelial dysfunction. In addition to NOS3, both NOS1 and NOS2 have been found to be S -nitrosated, but the evidence for dynamic regulation of those NOS isoforms by this process is less complete [ citation needed ] . In addition, both NOS1 and NOS2 have been shown to form ferrous-nitrosyl complexes in their heme prosthetic groups that may act partially to self-inactivate these enzymes under certain conditions [ citation needed ] . The rate-limiting step for the production of nitric oxide may well be the availability of L -arginine in some cell types. This may be particularly important after the induction of NOS2.
However, on introduction of a specific metabolite, usually a substrate, the concentration of the enzyme quickly increases. The metabolite that initiates the appearance of the enzyme is called an inducer. Inducible enzymes are the products of genes that are selectively expressed; these genes are referred to as inducible genes. One of the first inducible enzymes to be intensively studied was β-galactosidase. Wild-type E. coli cells metabolize glucose and will metabolize only the glucose even if lactose, another sugar, is also present.