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Thyroid hormones inhibit the Ca2+ calmodulin-induced activation of myosin light chain kinase
M Hagiwara1, S Mamiya, M Ochiai
1Department of Molecular and Cellular Pharmacology, Mie University School of Medicine, Japan.
Abstract:
L-Thyroxine (T4) and L-triiodothyronine (T3) specifically, inhibited myosin light chain kinase (MLC-kinase) from various tissues whereas inhibitory effects of T4 and T3 on other protein kinases such as protein kinase C, cAMP-dependent protein kinase, casein kinase I, casein kinase II and calmodulin kinase II were much weaker. T4 was a more potent inhibitor of MLC-kinase than T3. Kinetic studies showed that T4 behaved as a competitive inhibitor of MLC-kinase toward calmodulin (CaM) and that Ki value was 2.5 microM. The activity of the catalytic fragment of MLC-kinase, which is active without CaM, was not inhibited by T4. 125I-T4 gel overlay revealed that CaM did not bind T4 but MLC-kinase had 125I-T4 binding activity. These observations suggest that T4 binds at or near CaM binding domain of MLC-kinase and inhibits CaM-induced activation of MLC-kinase.
Insights
Thyroid hormones L-Thyroxine (T4) and L-triiodothyronine (T3) inhibit myosin light chain kinase (MLC-kinase). T4 is a potent inhibitor, binding near the calmodulin domain to block MLC-kinase activation.
Area of Science:
- Biochemistry
- Molecular Biology
- Endocrinology
Background:
- Thyroid hormones, L-Thyroxine (T4) and L-triiodothyronine (T3), play crucial roles in cellular metabolism and function.
- Myosin light chain kinase (MLC-kinase) is a key enzyme regulating muscle contraction and other cellular processes.
- The interaction between thyroid hormones and specific protein kinases is an area of ongoing research.
Purpose of the Study:
- To investigate the inhibitory effects of T4 and T3 on MLC-kinase.
- To determine the mechanism by which thyroid hormones affect MLC-kinase activity.
- To compare the potency of T4 and T3 as inhibitors of MLC-kinase.
Main Methods:
- Enzyme inhibition assays using MLC-kinase from various tissues.
- Kinetic analysis to determine inhibition type and binding constants.
- 125I-T4 gel overlay assays to assess direct binding interactions.
Main Results:
- T4 and T3 specifically inhibited MLC-kinase, with T4 being more potent than T3.
- T4 acted as a competitive inhibitor with respect to calmodulin (CaM), with a Ki of 2.5 microM.
- T4 did not inhibit the catalytic fragment of MLC-kinase, indicating CaM dependence for inhibition.
- Direct binding of T4 to MLC-kinase was observed, localized near the CaM binding domain.
Conclusions:
- Thyroid hormones, particularly T4, can directly inhibit MLC-kinase activity.
- The inhibition mechanism involves T4 binding to MLC-kinase near the CaM binding site, preventing CaM-induced activation.
- These findings reveal a novel regulatory pathway for MLC-kinase involving thyroid hormones.