Characterization of the interaction of parathyroid hormone with the mitochondrial ATPase

Abstract

Parathyroid hormone (PTH) has been shown to bind specifically to the β subunit of the mitochondrial ATPase on nitrocellulose blots. We have now examined this interaction further, using intact mitochondria, submitochondrial particles, and the purified F 1 ATPase. With intact mitochondria, 1 μ m concentrations of PTH and its biologically active 1–34 fragment activate the ATPase about 3-fold. This effect was reduced to a 1.4-fold activation with 3–34 and 7–34 fragments of the hormone, and oxidized PTH gave no detectable activity. Activation could only be observed below pH 7. PTH had no significant effect on the activity of the purified enzyme or on submitochondrial particles. However, specific binding of an iodinated PTH analog, [Nle 8,18-Tyr 34] bPTH (1–34) amide, was found with submitochondrial particles and the purified ATPase. Binding affinity with the purified enzyme was about 10 −3 that of the plasma membrane receptor, and the molar stoichiometry was close to 1:1 (PTH:intact enzyme). With submitochondrial particles the affinity was about 10-fold higher than with the purified enzyme. This binding was further examined with PTH derivatives and fragments, and compared to that seen in the plasma membrane receptor. Oxidation of methionine 18 in PTH reduced the affinity about 50%, oxidation of methionine 8 reduced the affinity 95%, and oxidation of both methionines further decreased affinity in both membranes and submitochondrial particles. However, when compared to the native hormone, the 3–34 and 7–34 PTH fragments had much higher affinity for the submitochondrial particles than for the plasma membranes. PTH also reduced chemical crosslinking of the ATP analog, p-fluorosulfonyl benzoyl 5′-adenosine, to the α subunit of this enzyme, but did not alter labeling of the enzyme with 3′- O-(4′-benzoyl) benzoyl ATP, suggesting that the hormone binds near a regulatory nucleotide binding site. Direct chemical crosslinking of PTH to the β-subunit of the enzyme was attained with a cleavable, photoactivate crosslinker, sulfosuccinimidyl 2-( p-azidosalicylamido) ethyl-1,3-dithiopropionate. The crosslinked protein was cleaved with cyanogen bromide and the labeled fragments were sequenced. The labeled fragments were found to be segments of the protein which have previously been implicated as being close to the noncatalytic ATP binding sites.