A homarine-synthesizing enzyme was found for the first time in cell-free extract from turban shell (Batillus cornutus) and the enzyme was purified 36.2-fold and characterized. Properties of the enzyme were as follows: substrates were picolinic acid (pyridine-2-carboxylic acid) and S-adenosyl-L-methionine; optimum temperature for the enzymic reaction was 25 degrees C; optimum pH for the enzymic reaction was 6.3; and the Km values for picolinic acid and S-adenosyl-L-methionine were calculated at 317 and 14.5 microM, respectively. Among pyridine carboxylic acids, only picolinic acid was methylated with S-adenosyl-L-methionine by this enzyme. The molecular weight of the enzyme was estimated to be 70,800. The enzyme activity was inhibited by heavy metal ions, S-adenosyl-L-homocysteine, adenosine, homocysteine, and sinefungin. Homarine, which is an osmotic pressure regulator, morphogen, etc.; is enzymatically synthesized by the methylation of picolinic acid with S-adenosyl-L-methionine and the enzyme activity may be controlled by S-adenosyl-L-homocysteine (reaction product) and its related compounds.