A 4-year-old with developmental delay, hypotonia, and intractable seizures has plasma glycine markedly elevated at 1200 μmol/L with a cerebrospinal fluid-to-plasma glycine ratio of 0.08. Which enzyme of the glycine cleavage system is most commonly deficient in this disorder?
- A P-protein (glycine decarboxylase) ✓
- B T-protein (aminomethyltransferase)
- C H-protein (lipoyl-containing protein)
- D L-protein (dihydrolipoamide dehydrogenase)
Correct answer: A. P-protein (glycine decarboxylase)
Explanation
Non-ketotic hyperglycinemia (glycine encephalopathy) most commonly results from P-protein (glycine decarboxylase) deficiency. Elevated CSF-to-plasma glycine ratio (>0.04) differentiates it from ketotic hyperglycinemia. T-protein deficiency is the second most common. H-protein and L-protein deficiencies are rare. The glycine cleavage system is the main catabolic pathway for glycine.
Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.