Protein carboxyl-O-methyltransferase activity in cultured C-1300 neuroblastoma cells

Protein carboxylmethyltransferase (PCM) has been identified in a variety of tissues derived from neural crest anlage, including in vivo C-1300 murine neuroblastoma (MNB). These observations have stimulated interest in further defining the role of PCM as a potential modulator of neoplastic cell behavior. The subcellular distribution and kinetic behavior of PCM have been characterized in a tissue culture line derived from the C-1300 murine neuroblastoma (clone NB41A3). The specific and total activities of PCM in the presence and absence of exogenous substrate were determined in subcellular fractions of MNB cells prepared by differential centrifugation. In the presence of exogenous substrate (+ gelatin), 40% of the total PCM activity was present in the 100,000 g supernatant fraction and 41% in the 800 g particulate fraction, whereas the higher specific activity of PCM was present in the 100,000 g supernatant fraction. Enzyme activity measured in the absence of gelatin, which reflects the concentration of endogenous methyl acceptor proteins in a cell fraction, was negligible. This activity represented less than 1.6 and 0.4% of the total PCM activity present in the 800 g particulate and 100,000 g soluble fractions respectively. Cytosolic PCM had an apparent Km of 13.9 x 10(-6) M for AdoMet and a Vmax of 33 pmoles per min per mg protein. Cytoplasmic PCM was inhibited competitively by S-adenosylhomocysteine (Ki equal 0.2 microM). These data demonstrate that the specific activity of PCM was greatest in the soluble component of subcellular fractions prepared from cultured MNB cells. This distribution pattern of PCM is similar to that observed in the C-1300 MNB tumor grown in situ and in non-malignant tissues. In contrast to the latter tissues, cultured MNB cells exhibited low PCM activity when assayed in the absence of exogenous substrate.