Mouse brain contains a high molecular weight nonfunctional protein with antigenic homology to dihydrofolate reductase

The cytosol of brain tissue from mature BDF₁ mice contains very little dihydrofolate reductase activity but it does contain a high molecular weight nonfunctional protein which cross-reacts in a radioimmunoassay for the active enzyme. Liver cytosal contains less of this high molecular weight cross-reacting protein and more of the functional dihydrofolate reductase. The cytosol from kidney contains very little of the high molecular weight cross-reacting protein, 95% of the immunoreactive proteins being the functional form of dihydrofolate reductase. The modification of dihydrofolate reductase into a nonfunctional form may be an intrinsic property of some cells and this finding could explain the variability in measuring the activity of this enzyme in brain tissue of mature animals.