Glycine N-methyltransferase (EC 2.1.1.20) catalyzes the methylation of glycine by S-adenosylmethionine to form sarcosine and S-adenosylhomocysteine. The enzyme was previously shown to be abundant in both the liver and pancreas of the rat, to consist of four identical monomers, and to contain tightly bound folate polyglutamates in vivo. We now report that the inhibition of glycine N-methyltransferase by (6S)-5-CH(3)-H(4)PteGlu(5) is noncompetitive with regard to both S-adenosylmethionine and glycine. The enzyme exhibits strong positive cooperativity with respect to S-adenosylmethionine. Cooperativity increases with increasing concentrations of 5-CH(3)-H(4)PteGlu(5) and is greater at physiological pH than at pH 9.0, the pH optimum. Under the same conditions, cooperativity is much greater for the pancreatic form of the enzyme. The V(max) for the liver form of the enzyme is approximately twice that of the pancreatic enzyme, while K(m) values for each substrate are similar in the liver and pancreatic enzymes. For the liver enzyme, at pH 7.0 half-maximal inhibition is seen at a concentration of about 0.2 microM (6S)-5-CH(3)-H(4)PteGlu(5), while at pH 9.0 this value is increased to about 1 microM. For the liver form of the enzyme, 50% inhibition with respect to S-adenosylmethionine at pH 7.4 occurs at about 0.27 microM. The dissociation constant, K(s), obtained from binding data at pH 7.4 is 0.095. About 1 mol of (6S)-5-CH(3)-H(4)PteGlu(5) was bound per tetramer at pH 7.0, and 1.6 mol were bound at pH 9.0. The degree of binding and inhibition were closely parallel at each pH. At equal concentrations of (6R,6S)- and (6S)-5-CH(3)-H(4)PteGlu(5), the natural (6S) form was about twice as inhibitory. These studies indicate that glycine N-methyltransferase is a highly allosteric enzyme, which is consistent with its role as a regulator of methyl group metabolism in both the liver and the pancreas.