A composition and method for lowing serum and plasma levels of methionine by oral administration. The composition includes a recombinant methioninase enzyme and a cofactor (pyridoxal-L-phosphate). Methods of use describe methods for treatment of cancer, including malignant melanoma, by oral administration of the methioninase composition. Methods for chronic suppressive therapy of melanoma and other cancers are described. Because reduction of plasma methionine levels is effective in treating other conditions, including diabetes and conditions associated with aging, the use of the methods described herein includes treatment of these and other conditions.