Beta-Amino Acids, Renal Transport Of

Connolly et al. (1979) stated that urinary taurine excretion values show three modes in normals, consistent with a polymorphic codominant 2-allele system regulating renal reabsorption. They estimated frequencies of 0.35 and 0.65 for the high and low reabsorption, respectively. Beta-alanine competitively inhibits reabsorption of taurine and BAIB (beta-aminoisobutyric acid; see 210100). Thus, the postulated system is probably homologous to the beta-amino acid renal transport system found in mice and rats. Taurine excretion is, on the average, low in the Down syndrome, suggesting to Connolly et al. (1979) that the gene encoding this system is on human chromosome 21. At HGM6 (Oslo, 1981), a tentative assignment of a locus for this function to chromosome 21 was made on the basis of dosage effect in Down syndrome. Goodman (1981) concluded that a polymorphic codominant pair of alleles, symbolized T(R) and T(S), for rapid and slow uptake of taurine, are the prime regulators of taurine reabsorption at the renal level. The subtlety of the difference (only about 20% in reabsorption between the two homozygous genotypes) makes taurine loading essential to rigorous demonstration. In Down syndrome subjects, four genotypes occur in frequencies suggesting that the gene is on chromosome 21. A correlation between primary taurine excretion and IQ in Down syndrome was observed by Thomas et al. (1965). Thus, the same variability in uptake may occur in brain cells. Goodman et al. (1980) claimed that taurine metabolism may be important in epilepsy. Taurine, like gamma-aminobutyric acid (GABA), is probably neuroinhibitory and serves a role in modulation of neurotransmission (Barbeau and Huxtable, 1978). Taurine accounts for more than half of the total free amino acids in brain and platelet. Variability in platelet taurine may be a useful way to examine this polymorphism. Goodman (1981) estimated that the frequency of the rapid absorption gene is about 0.338.