The monomer of pyruvate kinase, subtype M₁, is both a kinase and a cytosolic thyroid hormone binding protein

Using a T7 expression system, the monomer of rat pituitary pyruvate kinase, subtype M₁ (PKM₁), was overexpressed in Escherichia coli and purified to homogeneity. The monomeric p58-M₁ has intrinsic enzymatic activity with a Vmax of 79 ± 20 units/mg and Km's for ADP and PEP of 1.43 ± 0.76 and 0.14 ± 0.07 mM, respectively. The monomer binds 3,3′,5-triiodo-L-thyronine (T₃) with Ka = 1.5 × 10⁷ M^-1. The order of analog specificity is L-T₃ > L-thyroxine > D-T₃ > 3′-isopropyl-3,5-diiodo-L-thyronine ≥ 3′,5′,3-triiodo-L-thyronine. In contrast, tetrameric PKM₁ lacks T₃ binding activity. The kinase activity of p58-M₁ is inhibited by T₃ and its analogs in a concentration-dependent manner with the order of inhibitory activity similar to that of binding activity. This inhibition, however, is reversed by the addition of fructose 1,6-bisphosphate. p58-M₁ is the second PK isoenzyme monomer to be identified as having thyroid hormone binding activity.