The saccharopine pathway (SacPath) and the pipecolate pathway (PipPath) catabolize lysine to α-aminoadipate. Although the PipPath has been highlighted as the prominent route operating in the brain, recent work has demonstrated that the SacPath plays a major role in lysine catabolism in the brain. The first two enzymatic steps of the SacPath involve the bifunctional enzyme α-aminoadipate semialdehyde synthase (AASS) harboring the lysine-ketoglutarate reductase (LKR) and the saccharopine dehydrogenase (SDH) domains that convert lysine to α-aminoadipate semialdehyde. Thereafter, the semialdehyde is converted to α-aminoadipate by α-aminoadipate semialdehyde dehydrogenase (AASADH). Mutations abolishing the enzymatic activities of LKR, SDH, and AASADH lead to the genetic diseases hyperlysinemia type I and II, and pyridoxine-dependent epilepsy (PDE), respectively. Hyperlysinemia type I accumulates lysine and causes a benign phenotype without clinical significance. Hyperlysinemia type II accumulates saccharopine, which leads to neuronal disorders and intellectual disability. PDE accumulates α-aminoadipate semialdehyde and its cyclic isomer piperideine-6-carboxylate, which binds pyridoxal 5'-phosphate, disturbs synapses, and causes seizures along with developmental disorders. Another genetic disease, glutaric aciduria type I (GA1), localizes just downstream of the SacPath and is caused by mutations abolishing the enzymatic activity of glutaryl-CoA dehydrogenase (GCDH). GA1 accumulates glutarate and 3-hydroxyglutarate, which are neurotoxic molecules that cause irreversible brain damage. Downregulation of LKR has been shown to reduce the metabolic flux through SacPath and alleviate PDE and GA1 symptoms. This review discusses the role of SacPath and its enzymes as potential targets for developing drugs to treat PDE and GA1, as well as other diseases.

In mitochondria, metabolic processes require the trafficking of solutes and organic molecules, such as amino acids. This task is accomplished by the Mitochondrial Carrier Family members (also known as SLC25), among which the SLC25A29 is responsible for the translocation of basic amino acids. In this regard, nitric oxide levels originated by the arginine mitochondrial catabolism have been shown to strongly affect cancer cells' metabolic status. Furthermore, the metabolic disease saccharopinuria has been linked to a mitochondrial dysregulation caused by a toxic intermediate of the lysine catabolism. In both cases, a reduction of the activity of SLC25A29 has been shown to ameliorate these pathological conditions. However, no detailed structural data are available on SLC25A29. In the present work, molecular modelling, docking and dynamics simulations have been employed to analyse the structural determinants of ligands recognition by SLC25A29 in the c-state. Results confirm and reinforce earlier predictions that Asn73, Arg160 and Glu161, and Arg257 represent the ligand contact points I, II, and III, respectively, and that Arg160, Trp204 and Arg257 form a stable interaction, likely critical for ligand binding and translocation. These results are discussed in view of the experimental data available for SLC25A29 and other homologous carriers of the same family.

We report a case of saccharopinuria with hyperammonemia and hypercitrullinemia in a Japanese woman who presented with elderly-onset epilepsy, progressive cognitive decline, and gait ataxia. Blood amino acid analysis revealed an increase in citrulline, cystine, and lysine levels, and urine amino acid analysis showed increased citrulline and cystine levels. Urine metabolomics revealed an increased saccharopine level, leading to the definitive diagnosis of saccharopinuria. In western blots of liver biopsy samples, normal citrin levels were observed, suggesting that adult-onset citrullinemia type 2 (CTLN2) was not present. In addition, decreased argininosuccinate synthetase (ASS) levels were observed, and ASS1 gene, a causative gene for citrullinemia type 1 (CTLN1), was analyzed, but no gene mutations were found. Because the causes of hypercitrullinemia were not clear, it might be secondary to saccharopinuria. Muscle biopsy findings of the biceps brachii revealed diminished cytochrome c oxidase (COX) activity, mitochondrial abnormalities on electron microscopy and p62- positive structures in immunohistochemical analyses. Saccharopinuria is generally considered a benign metabolic variant, but our case showed elevated lysine and saccharopine levels causing ornithine circuit damage, mitochondrial dysfunction, and autophagy disorders. This may lead to so far unknown neurological disorders.

Amino acid catabolism is frequently executed in mitochondria; however, it is largely unknown how aberrant amino acid metabolism affects mitochondria. Here we report the requirement for mitochondrial saccharopine degradation in mitochondrial homeostasis and animal development. In Caenorhbditis elegans, mutations in the saccharopine dehydrogenase (SDH) domain of the bi-functional enzyme α-aminoadipic semialdehyde synthase AASS-1 greatly elevate the lysine catabolic intermediate saccharopine, which causes mitochondrial damage by disrupting mitochondrial dynamics, leading to reduced adult animal growth. In mice, failure of mitochondrial saccharopine oxidation causes lethal mitochondrial damage in the liver, leading to postnatal developmental retardation and death. Importantly, genetic inactivation of genes that raise the mitochondrial saccharopine precursors lysine and α-ketoglutarate strongly suppresses SDH mutation-induced saccharopine accumulation and mitochondrial abnormalities in C. elegans Thus, adequate saccharopine catabolism is essential for mitochondrial homeostasis. Our study provides mechanistic and therapeutic insights for understanding and treating hyperlysinemia II (saccharopinuria), an aminoacidopathy with severe developmental defects.