[Evidence and assessment of methionine synthetase activity in normal and pathologic cells of the blood and of bone-marrow]

Vitamin B12-dependent methyltetrahydrofolate: homocysteine methyltransferase activity in normal and leukemic human hematopoietic cells

The cyanogen bromide method was applied to the assay of vitamin B12-dependent methyltetrahydrofolate:homocysteine methyltransferase activity in normal and leukemic human hematopoietic cells. Normal peripheral lymphocytes and leukemia cells of lymphoid origin wuch as CLL and ALL, contained higher levels of enzyme activity than did normal human bone marrow cells. Normal granulocytes and leukemia cells of myeloid origin, such as CML in the chronic phase and AML, contained lower enzyme activity. Leukemia cells of CML in blast crisis showed higher mean activity than in the chronic phase of the disease.

Studies on N5-methyltetrahydrofolate-homocystein methyltransferase in normal and leukemia leukocytes

A cobalamin-dependent N5-methyltetra-hydrofolate-homocysteine methyltransferase (methyl-transferase) was demonstrated in unfractioned extracts of human normal and leukemia leukocytes. Activity was substantially reduced in the absence of an added cobalamin derivative. Presumably, this residual activity reflects the endogeneous level of holoenzyme. Enzyme activity was notably higher in lymphoid cells than in myeloid cells. Thus, mean specific activities (+/-SD) were: chronic lymphocytic leukemia lymphocytes, 2.15+/-1.16; normal lymphocytes, 0.91+/-0.59; normal mature granulocytes, 0.15+/-0.10; chronic myelocytic leukemia granulocytes, barely detectable activity. Properties of leukocytes enzymes resembled those of methyltransferases previously studied in bacteria and other animal cells. Granulocytes and chronic myelocytic leukemia cells contain a factor or factors that inhibits Escherichia coli enzyme. The data suggest that the prominence of this cobalamin-dependent enzyme in lymphocytes and other mononuclear cell types may be related to their potential for cell division.