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Purification and characterization of a smooth muscle myosin phosphatase from turkey gizzards
Abstract:
A phosphoprotein phosphatase that dephosphorylates smooth muscle myosin has been purified to apparent homogeneity from turkey gizzards. Smooth muscle phosphatase (SMP) IV has a molecular weight of 150,000 as determined by gel filtration on a Sephadex G-200 column and is composed of two subunits (Mr = 58,000 and 40,000). Although it is active toward a number of proteins, its activities toward the contractile proteins, intact myosin, heavy meromyosin, and isolated myosin light chains are higher than its activities toward phosphorylase alpha, histone IIA, and phosphorylase kinase. SMP-IV preferentially dephosphorylates the beta-subunit of phosphorylase kinase. The properties of the enzyme have been studied using heavy meromyosin, a soluble chymotryptic fragment of myosin, and isolated myosin light chains as substrates. SMP-IV has high affinity for both substrates and is optimally active at neutral pH. Divalent cations, Ca2+ and Mg2+, activate the dephosphorylation of heavy meromyosin but inhibit the activity toward myosin light chains. Low concentrations of ATP (1-5 mM) activate SMP-IV but concentrations higher than 5 mM are inhibitory. Inhibition of 50% of the activity of the enzyme by NaF and PPi requires concentrations higher than 10 mM. Rabbit skeletal muscle heat stable inhibitor-2 has no effect on the activity of SMP-IV toward heavy meromyosin, myosin light chains, and phosphorylase alpha.
Insights
A novel phosphoprotein phosphatase, smooth muscle phosphatase (SMP) IV, was purified from turkey gizzards. This enzyme dephosphorylates smooth muscle myosin and related contractile proteins with high affinity.
Area of Science:
- Biochemistry
- Molecular Biology
- Enzymology
Background:
- Smooth muscle contraction is regulated by the phosphorylation state of myosin.
- Phosphoprotein phosphatases play a crucial role in regulating myosin dephosphorylation.
- Understanding these enzymes is key to deciphering muscle function.
Purpose of the Study:
- To purify and characterize a novel phosphoprotein phosphatase from turkey gizzards.
- To investigate the substrate specificity and kinetic properties of the purified enzyme.
- To elucidate the enzyme's role in regulating smooth muscle myosin.
Main Methods:
- Purification of phosphoprotein phosphatase using gel filtration (Sephadex G-200).
- Enzyme activity assays using various protein substrates including myosin, heavy meromyosin, and myosin light chains.
- Characterization of enzyme properties including molecular weight, subunit composition, pH optimum, and effects of cations and ATP.
Main Results:
- Purified a phosphoprotein phosphatase, designated smooth muscle phosphatase (SMP) IV, with a molecular weight of 150,000 Da, composed of two subunits.
- SMP-IV exhibited higher activity towards contractile proteins (myosin, heavy meromyosin, myosin light chains) than other tested proteins.
- The enzyme showed high affinity for heavy meromyosin and myosin light chains, optimal activity at neutral pH, and differential regulation by divalent cations and ATP.
Conclusions:
- SMP-IV is a distinct phosphoprotein phosphatase with a preference for smooth muscle myosin and its fragments.
- The enzyme's properties suggest a significant role in the regulation of smooth muscle contraction.
- Further studies are warranted to fully understand its physiological function and regulatory mechanisms.