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André Hallen, Arthur J. L. Cooper, Letter to the Editor: μ-Crystallin/CRYM functions as a ketimine-reducing enzyme and plays a role in thyroid hormone bioavailability due to strong inhibition/regulation by thyroid hormones, The Journal of Clinical Endocrinology & Metabolism, Volume 100, Issue 1, 1 January 2015, Pages L15–L16, https://doi.org/10.1210/jc.2014-3963
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We have read with interest the recent article by Serrano et al (1) regarding the thyroid hormone-binding protein μ-crystallin (CRYM). Serrano et al state, “Recently, the spectrum of functions of this hormone-binding protein has been broadened since a recent report considers μ-crystallin as a versatile enzyme with a reductase activity able to accommodate various substrates such as ketimine, thyroid hormones or alanine dehydrogenase ligands pyruvate and alanine.” We wish to comment on this statement.
Serrano et al (1) state, “As the main function of μ-crystallin is proposed to be transport of intracellular thyroid hormone…. ” We also wish to comment on this statement. We would like to clarify that CRYM has numerous functions; however, it is primarily an enzyme as noted above. Its function in intracellular thyroid hormone transport is related to its role as a ketimine reductase. The level of free thyroid hormone in the cytosol will depend in part on the strength of its binding to CRYM. As noted above, T3 acts as a strong inhibitor/regulator of CRYM enzyme activity. Conversely, binding of cyclic ketimine substrate will compete with binding of T3.