Quantitation of serum lactate dehydrogenase-5 with monoclonal antibodies.

Spleen cells from BALB/cJ mice which had been immunized with human lactate dehydrogenase-1 (LDH-1) were fused with SP2/0-Ag14. Two hybridomas were produced which recognized the antigen. Competitive RIA revealed that one antibody ('Smit-LDH') recognized the H subunit of LDH while the other ('Hem-LDH') recognized both H and M subunits of LDH. With the use of these antibodies we developed an assay for LDH-5 activity in which serum is incubated for 30 min at room temperature with the two antibodies ('Smit-LDH' and 'Hem-LDH' in the ratio 64:1.3, micrograms/ml) immobilized on latex beads to extract LDH-1 through LDH-4. After centrifugation, the LDH activity of the supernatant is measured and represents LDH-5 activity. Latex beads coated with bovine serum albumin were used as control. The LDH-5 activity as determined by our assay correlated well (r = 0.98) with the values obtained by an electrophoresis method. There was no interference due to LDH-1 through LDH-3 up to 3,000 U/l and LDH-4 up to 350 U/l. Serum samples with total LDH activity above 1,000 U/l were appropriately diluted in order to avoid interference by LDH-4. Use of these monoclonal antibodies allows precise, rapid and direct measurement of LDH-5 activity in serum.