Inhibits lactate dehydrogenase (LDH)-mediated conversion of lactate to pyruvate in mitochondria by competing with mitochondrial LDH for binding to NAD(+). Also inhibits cellular lactate utilization. Interacts with lactate dehydrogenases LDHA and LDHB; interaction with mitochondrial LDH leads to inhibition of lactate dehydrogenase activity, preventing conversion of lactate to pyruvate. Detected in brain, kidney and liver (at protein level). Enhanced global lactate metabolism with elevated glucose consumption, increased ATP production and decreased perigonadal fat pad weight and fat/body weight ratio (PubMed:33406399). Elevated serum and secreted lactate levels (PubMed:33406399). Conditional knockout in astrocytes results in initiation of gliomagenesis and development of neurological symptoms that include seizure, ataxia and/or paralysis at 55-65 weeks of age (PubMed:33406399).