Atypical nonketotic hyperglycinemia with a defective glycine transport system in nervous tissue.

The biochemical characteristics of a patient with most of the clinical symptoms of nonketotic-hyperglycinemia have been studied. Despite the extremely low plasma/cerebrospinal fluid glycine ratio of the patient, typical of the nonketotic syndrome, at autopsy we found no enhancement of glycine levels in brain and glycine cleavage enzyme activities in the brain and liver that were similar to those of a control newborn. In order to ascertain the basic defect responsible for glycine accumulation in the cerebrospinal fluid and for the neurological damage, we examined the possible existence of altered glycine transport systems in nervous cells. Our findings indicate that (1) low and high affinity, Na+-dependent transport systems for glycine can be detected in both rat and control human postmortem tissue and (2) the low affinity glycine transport system is absent in the brain of the patient, whereas an alteration of transport systems occurs in the patient's spinal cord, probably involving the high-affinity component. These alterations could account for the clinical features of the patient. Since the feasibility of determining glycine transport parameters in postmortem tissue has been established, we think it would be of interest to investigate such systems in cases where the etiology of hyperglycinemia is not clear.