Inhibits lactate dehydrogenase (LDH)-mediated conversion of lactate to pyruvate in mitochondria by competing with mitochondrial LDH for binding to NAD(+) (PubMed:33406399). Also inhibits cellular lactate utilization (PubMed:33406399). Interacts with lactate dehydrogenases LDHA and LDHB; interaction with mitochondrial LDH leads to inhibition of lactate dehydrogenase activity, preventing conversion of lactate to pyruvate. Expressed in brain (at protein level) (PubMed:33406399). Expressed at lower levels in glioblastomas than in normal brain tissue (at protein level) (PubMed:33406399). Recombinant MP31 administered intraperitoneally penetrates the blood-brain barrier and inhibits mice glioblastoma xenografts without neurological toxicity, suggesting a potential role in gliobastoma treatment.