On rat renal aminolevulinate transport and metabolism in experimental Fanconi syndrome.

Hereditary tyrosinemia, an autosomal recessive disease of human infants, is characterized by severe liver disease, a renal Fanconi syndrome, and urinary excretion of large quantities of both aminolevulinate (ALA) and succinylacetone (SA). The latter is a metabolic end-product of tyrosine catabolism in affected individuals, produced by both liver and kidney, and is a potent inhibitor of aminolevulinate dehydratase (ALAD) in liver. This inhibition has been assumed to result in release of large amounts of aminolevulinate from liver into the circulation, with subsequent urinary excretion. In the present report we examine the effects of succinylacetone on rat renal cortical tubular handling of ALA and the relationship to tubular heme content, demonstrating a marked impairment of each. In contrast, maleic acid was found to have no effect on either renal ALAD or heme content. Thus, we conclude that renal handling of ALA in SA-treated rat renal cortex may indicate a contribution by the kidney to the increased net ALA excretion observed in hereditary tyrosinemia.

delta-Aminolevulinic acid dehydratase: is there a form unique to renal cortex?

Succinylacetone (SA) is known to be a potent inhibitor of delta-aminolevulinic acid dehydratase (ALAD) in the liver. We have examined the effects of SA on the rat renal cortical enzyme, our observations indicating very different behaviour of renal versus hepatic ALAD with SA treatment. While the temperature response of ALAD in both tissues was similar, addition of 4 mmol/l SA inhibited liver ALAD at 37 and 55 degrees C and enhanced renal ALAD activity 2- to 3-fold at each temperature. This increase in renal ALAD was progressive with SA concentrations form 1 to 10 mmol/l. A pH titration curve for both liver and kidney ALAD showed the hepatic enzyme to have a single pH optimum, while the renal enzyme had two, each of which was distinct from that in liver. Kinetic studies with and without 4 mmol/l SA over a 50-fold ALA concentration range indicated SA-induced enhancement of renal ALAD over the entire range at both pH optima. Using 14C-labelled ALA, we have confirmed these observations made on the basis of a colorimetric assay for PBG, the enzyme product. We conclude that renal ALAD may be a different molecular species from the liver enzyme. Further studies may clarify the significance of these observations to renal heme synthesis.

Renal Fanconi syndrome: developmental basis for a new animal model with relevance to human disease.

Using succinylacetone (SA), a metabolite of tyrosine excreted in excess by infants and children with hereditary tyrosinemia and the renal Fanconi syndrome (FS), we have investigated developmentally-related membrane transport events leading to emergence of the generalized renal tubular dysfunction seen in human FS. SA was found to impair sugar and amino acid uptake by both newborn renal tubules and 7-day renal brush-border membrane vesicles (BBMV). This impairment by SA was due in part to a slowing of substrate cotransport rate of 22Na+-entry into BBMV. Concentration-dependent uptake studies indicated SA inhibited the newborn high-affinity transport systems for sugars and amino acids. SA also caused an increase in membrane fluidity and a shift in the thermotropic transition temperature. The demonstrated dual nature of SA's effect on membrane fluidity and O2 consumption, together with the relative contribution of each component to SA-induced transport impairment helps to provide a basis for an understanding of the age-related increases in glucosuria, aminoaciduria and natriuria seen in infants with FS.

Effects of succinylacetone on the uptake of sugars and amino acids by brush border vesicles.

Infants with hereditary tyrosinemia excrete succinylacetone (SA) in their urine, and suffer from a reversible renal Fanconi syndrome with glycosuria and hyperaminoaciduria. Thus, we have examined the effects of 4 mM SA on rat renal brush border membrane vesicle uptake of sugars and amino acids. SA, unlike sodium maleate, significantly inhibits Na+-dependent vesicular sugar and amino acid uptake. 22Na-uptake, as well as membrane fluidity of the vesicles, are also affected by SA. Inhibition of glycine uptake by SA is reversible and competitive in nature, while alpha-CH3-D-glucoside uptake is non-competitively affected. We conclude, therefore, that SA has a more complex action on the rat renal tubule than sodium maleate, and is likely a much more physiologic model for study of the human renal Fanconi syndrome.