enzymes that are present in cells at all times (in contrast to induced enzymes, whose synthesis depends on the adaptation of the organism to a specific substrate).

The terms “constitutive enzyme” and “induced enzyme” were initially applied to bacterial enzymes whose biosynthesis was studied in relation to the composition of the nutrient medium, or substrate. The two types differ quantitatively rather than qualitatively: most induced enzymes are formed in insignificant quantities without the addition of an inducer, whereas the amount of many constitutive enzymes increases when the specific substrate is added. Induced enzymes can become constitutive by mutation (for example, a mutant was found in E. coli that produced the induced enzyme β-galactosidase in the total absence of an inducer). Crystalline preparations of the enzyme, formed both inductively and constitutively (in a mutant), are identical in all respects. According to the unitary theory, both classes of enzymes are formed by the same mechanism as a result of induction, but only in constitutive enzymes do normal metabolites function as inducers.