1. Methionine adenosyltransferase (ATP:l-methionine-S-adenosyl transferase, EC 22.214.171.124), cystathionine β-synthase [l-serine hydro-lyase (adding homocysteine), EC 126.96.36.199] and cystathionine γ-lyase [l-cystathionine cysteine-lyase (deaminating), EC 188.8.131.52] activities were found only in the cytosol fraction of rat liver cells. None was found in the mitochondrial or endoplasmic reticulum fractions as judged by the distribution of marker enzymes on a density gradient after centrifugation of the cytoplasmic fraction of a liver homogenate, or in a preparation of liver cell nuclei.
2. Polymorphs, lymphocytes (with admixed monocytes) and mixed bone marrow white cells contained no methionine adenosyl transferase, cystathionine β-synthase or cystathionine γ-lyase activities.
3. The possible bearing of these results on the problem of abnormal cystine storage in cystinosis is briefly discussed.