Proteins of the LctP family have been found in Gram-negative and Gram-positive bacteria as well as archaea. One member of the family, from E. coli, is the lactate:H+ symporter (Dong et al., 1993). Two closely related paralogues have been found encoded within the E. coli genome, and one of these has been shown to be the glycolate uptake permease (Nunez et al., 2001). Both permeases transport the same acids (L- and D-lactate as well as glycolate), but their physiological substrates differ. The lactate permease transports both L- and D-lactate while the glycolate permease transports glycolate under physiological conditions. Regulatory effects determine the substrates transported in vivo.

Transport reactions catalyzed by functionally characterized members of the LctP family include:

D- or L-lactate or glycolate (out) + H⁺ (out) → D- or L-lactate or glycolate (in) + H⁺ (in).