Mu-crystallin (CRYM 16p13.11-p12.3. in human) is a lens structural protein in diurnal marsupials and an enzyme in mammalian possible involved in the potassium-ion recycling system; in addition it is a mammalian homologue of Agrobacterium ornithine cyclodeaminase..
In mammalian forebrain ketimine reductase was identified as mu-crystallin. It was also seen that CRYM encoded "adenine dinucleotide-nicodinamide phosphate (NADPH) -regulated thyroid hormone-binding protein (THBP)"; this identifies a new role for thyroid hormones in regulating mammalian amino acid metabolism.
There are many publications that correlate these enzyme activities to various pathological events.
Analysis of human tissues detected abundant expression of CRYM in heart, brain, skeletal muscle, and kidney, lower expression in lung and liver.
https://www.researchgate.net/post/Does_anyone_know_anything_about_the_metabolism_of_proline_in_Tuberous_Sclerosis
It's an interesting question, especially with the recognition that proline is an important regulatory molecule. The paper by Kim et al. (PNAS, 1992) showed expression of mu-crystalline in human heart, brain, skeletal muscle and kidney I n northerns, but they did not mention specific OCD enzymatic activities in these tissues. In fact, the mu-crystalline in kangaroo lens has no assayable OCD activity. The expression (northerns) shown in human tissue is a mystery. I do know, however, that in cultured cells, proline auxotrophy is accompanied by deficiency of both ornithine aminotransferase and P5C synthase. Deficiency of OCD would have to be a concomitant in these aucxotrophs if the proline from OCD is physiologically significant. Nevertheless, OCD could contribute to proline synthesis in specific tissues under stress or pathologic conditions. The canonical pathway for proline synthesis is markedly increased by oncogenes i.e. c-MYC (Liu W et al., PNAS 2012). This deserves further investigation but by monitoring enzyme activity and not only by northerns or westerns.
Recently it was shown that mu-crystallin enters the pipecolate pathway (metabolism of lysine). Prof Phang years ago showed that the proline affects the metabolism of lysine. These two facts might be useful to put some light on what is being discussed?
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3838446/
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Using radioisotopically labeled P5C, we showed that proline inhibitsf P5C reductase activity. However, we never studied P2C reductase.activity.
Kinetic data obtained at neutral pH suggests that ketimine reductase/CRYM plays a major role as a P2C/Pyr2C reductase and that AECK is not a major substrate at this pH. Thus, ketimine reductase is a key enzyme in the pipecolate pathway, which is the main lysine degradation pathway in the brain.
Article Insights into Enzyme Catalysis and Thyroid Hormone Regulatio...
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