cDNA clones, containing the entire coding region of rat L-type pyruvate kinase, were isolated and their nucleotide sequences were determined by the dideoxy-chain-termination method. The predicted coding region, which spans 543 amino acids, established the complete amino acid sequence of the L-type isozynie of pyruvate kinase for the first time. The deduced amino acid sequence ofthe L type has one phosphorylation site in its amino terminus and shows about 68% and 48% homologies with MI-type pyruvate kinase of chicken and yeast pyruvate kinase respectively. Domain A exhibits higher homology than domains B and C. The residues in the active site of the L-type enzyme of rats, lying between domains B and A2, are rather different from those of the M,-type enzyme of chickens, but other residues constituting the active site are identical with those of the chicken M , type except for one amino acid substitution.Four isozymes of pyruvate kinase, an important glycolytic enzyme, have been found in mammals. These are named the L type, R type, M I type and M 2 type and differ in enzymatic properties as well as in regulation of gene expression [ l -41. We have purified the four isozymes from rat tissues and characterized them [S]. They all consist of four identical subunits of about 60000 D a and all but the M1 type are allosterically regulated. The L type is very similar, but not identical, to the R type in amino acid composition, peptide fingerprint patterns and immunological properties [I, 6, 71.Both isozymes are also phosphorylated by cyclic-AMP-dependent protein kinase, which results in their inactivation [8,, in fact, showed that the two isozymes are encoded by the same gene but have distinct messenger RNAs. A similar relation between the M I and Mz types has been reported [14, 151. However, little information is available on the structures of protein and gene of these isozymes except for the M1 typeThe M 2 type is the only isozyme detected in early fetal tissues [2,19]. In adults, pyruvate kinase isozymes show tissuespecific distributions [2,4, 191; the L type is the major isozyme in liver, the M I type is the main form in muscle, heart and brain, and the R type is detected only in red cells. Transition from the fetal to adult type occurs in the late fetal to early postnatal period. Thus, expression of pyruvate kinase isozymes is tissue-specific and is regulated developmentally. This regulation is probably at the level of transcription. In addition, expression of the hepatic L-type pyruvate kinase gene is regulated by insulin and glucagon [20, 211.