Inosine monophosphate dehydrogenase activity in acute leukaemia

Inosine monophosphate dehydrogenase (IMPD) is an important enzyme in de-novo purine synthesis. The level of IMPD activity has been suggested to determine whether acute leukaemia cells proliferate (if the activity is high) or differentiate (if IMPD activity is low). IMPD activity measured by the conversion of inosine monophosphate to xanthine monophosphate ranged from 12.5 to 87.0 (mean 49.4) pmol/h/10 6 cells in normal bone marrow. The levels were significantly raised in AML (range 14–374, mean 184 pmol/h/10 6 cells) and ALL (range 65–228, mean 172 pmol/h/10 6 cells). Normal tonsillar (B) lymphocytes showed higher levels (range 78–159, mean 110 pmol/h/10 6 cells) than resting peripheral blood T lymphocytes (range 8.8–51.2, mean 28.1 pmol/h/10 6 cells). In CLL, the results (range 19–173, mean 64.3 pmol/h/10 6 cells) were comparable to those of normal tonsillar B lymphocytes. IMPD levels could be related to cell cycle in PHA-stimulated lymphocytes, since IMPD activity increased in parallel with increase in DNA synthesis measured by labelled thymidine incorporation. On the other hand, IMPD activity did not correlate with the proportion of proliferating cells measured on a FACS sorter in either AML or ALL or in normal tonsillar B cells. We conclude that IMPD levels are higher in B than T lymphocytes and in acute leukaemia blasts compared to more differentiated mixed bone marrow cells. The results do not suggest, however, that IMPD assay will be of value in differentiation of the various subtypes of acute leukaemia or of malignant haemopoietic cells from the equivalent normal cell at the same level of differentiation.