Choline/ethanolamine kinase (ChoK/EtnK) exists as at least three isoforms (alpha1, alpha2 and beta) in mammalian cells. The physiological significance for the existence of more than one form of the enzyme, however, remains to be determined. In the present study, we examined the expression and distribution of the isoforms in mouse tissues using isoform-specific cDNA probes and polyclonal antibodies raised against each N-terminal peptide sequence. Both Northern- and Western-blot analyses indicated that either the alpha (alpha1 plus alpha2) or the beta isoform appeared to be the ubiquitously expressed enzyme. The mRNA abundance for the alpha isoform was highest in testis, whereas that for the beta isoform was relatively high in heart and liver. While the native form of each isoform was reported to consist of either homodimers or homotetramers, our immunotitration studies clearly indicated that a considerable part of the active form of the enzyme consists of alpha/beta hetero-oligomers, with relatively small parts of activity expressed by alpha/alpha and beta/beta homo-oligomers. This is the first experimental evidence for the presence of heteromeric ChoK/EtnK in any source. Thus our results strongly suggested that the activity of ChoK/EtnK in the cell is controlled not only by the level of each isoform but also by their combination to form the active oligomer complex. Carbon tetrachloride (CCl(4)) was shown to induce ChoK activity 2-4-fold in murine liver. Our analysis for the mechanism involved in this induction revealed that the responsible isoform for CCl(4) was alpha, not beta. The level of alpha mRNA was strongly induced in mouse liver, which resulted in a sustained increase in the amount of the alpha isoform. Consequently, the composition of alpha/alpha homo-oligomers came to represent up to 80% of the total active molecular form of ChoK in CCl(4)-induced liver, whereas it was less than 20% in normal uninduced liver.