Characteristics of a lysosomal membrane transport system for tyrosine and other neutral amino acids in rat thyroid cells

The lysosomal membrane-export of metabolites and beyond

Lysosomes are degradative organelles in eukaryotic cells mediating the hydrolytic catabolism of various macromolecules to small basic building blocks. These low-molecular-weight metabolites are transported across the lysosomal membrane and reused in the cytoplasm and other organelles for biosynthetic pathways. Even though in the past 20 years our understanding of the lysosomal membrane regarding various transporters, other integral and peripheral membrane proteins, the lipid composition, but also its turnover has dramatically improved, there are still many unresolved questions concerning key aspects of the function of the lysosomal membrane. These include a possible function of lysosomes as a cellular storage compartment, yet unidentified transporters mediating the export such as various amino acids, mechanisms mediating the transport of lysosomal membrane proteins from the Golgi apparatus to lysosomes, and the turnover of lysosomal membrane proteins. Here, we review the current knowledge about the lysosomal membrane and identify some of the open questions that need to be solved in the future for a comprehensive and complete understanding of how lysosomes communicate with other organelles, cellular processes, and pathways.

A dileucine signal situated in the C-terminal tail of the lysosomal membrane protein p40 is responsible for its targeting to lysosomes

Transport of newly synthesized lysosomal membrane proteins from the TGN (trans-Golgi network) to the lysosomes is due to the presence of specific signals in their cytoplasmic domains that are recognized by cytosolic adaptors. p40, a hypothetical transporter of 372 amino acids localized in the lysosomal membrane, contains four putative lysosomal sorting motifs in its sequence: three of the YXXphi-type (Y(6)QLF, Y(106)VAL, Y(333)NGL) and one of the [D/E]XXXL[L/I]-type (EQERL(360)L(361)). To test the role of these motifs in the biosynthetic transport of p40, we replaced the most critical residues of these consensus sequences, the tyrosine residue or the leucine-leucine pair, by alanine or alanine-valine respectively. We analysed the subcellular localization of the mutated p40 proteins in transfected HeLa cells by confocal microscopy and by biochemical approaches (subcellular fractionation on self-forming Percoll density gradients and cell surface biotinylation). The results of the present study show that p40 is mistargeted to the plasma membrane when its dileucine motif is disrupted. No role of the tyrosine motifs could be put forward. Taken together, our results provide evidence that the sorting of p40 from the TGN to the lysosomes is directed by the dileucine EQERL(360)L(361) motif situated in its C-terminal tail.

Molecular physiology and pathophysiology of lysosomal membrane transporters

In contrast to lysosomal hydrolytic enzymes, the lysosomal membrane remains poorly characterized. In particular, although the genetic study of cystinosis and sialic acid storage disorders led to the identification of two lysosomal transporters for cystine and sialic acids, respectively, ten years ago, most transporters responsible for exporting lysosomal hydrolysis products to the cytosol are still unknown at the molecular level. However, two lines of investigation recently started to fill this gap in the knowledge of lysosomal biology. First, novel proteomic approaches are now able to provide a reliable inventory of lysosomal membrane proteins. On the other hand, a novel functional approach based on intracellular trafficking mechanisms allows direct transport measurement in whole cells by redirecting recombinant lysosomal transporters to the cell surface. After surveying the current state of knowledge in this field, the review focuses on the sialic acid transporter sialin and shows how recent functional data using the above whole-cell approach shed new light on the pathogenesis of sialic acid storage disorders by revealing the existence of a residual transport activity associated with Salla disease.

In vitro characterization of the thyroidal uptake of O-(2-[18F]fluoroethyl)-l-tyrosine

OBJECTIVES

Positron emission tomography (PET) using O-(2-[(18)F]fluoroethyl)-l-tyrosine (FET) has been successfully employed in the diagnostic workup of brain tumors. Knowledge on the mechanisms of the uptake of radiolabeled amino acids into thyroidal tissues and well-differentiated thyroid carcinomas is limited. We therefore studied several factors potentially governing the uptake of FET in the rat thyroid cell line FRTL-5 in comparison with thyroid tumor cell lines of human origin.

METHODS

FET uptake was determined in thyroid-stimulating hormone (TSH)-stimulated and TSH-deprived FRTL-5 cells, as well as in the cell lines U-138 MG (human glioblastoma), Onco DG-1 (human papillary thyroid carcinoma) and ML-1 (human follicular thyroid carcinoma). The TSH responsiveness of cells was measured by the incubation of TSH-treated and untreated control cells with 2-[(18)F]fluoro-2-deoxyglucose (FDG). All cellular tracer uptake values were related to total protein mass and expressed as percentage per milligram. For countertransport studies, FRTL-5 cells were exposed to 10-300 microM tyrosine methyl ester. TSH-stimulated and TSH-deprived FRTL-5 cells were incubated with 100 kBq/ml FET for 20 min. 2-Aminobicyclo-[2,2,1]heptane-2-carboxylic acid (BCH), alpha-(methylamino)-isobutyric acid, L-serine and tryptophan were used as competitive inhibitors of FET uptake. All inhibition experiments were repeated with the human thyroid carcinoma cell lines to obtain comparative FET uptake values.

RESULTS

The FET uptake was 155+/-30%/mg in FRTL-5 cells (n=6), 108+/-14%/mg in U-138 MG cells (n=6), 194+/-60%/mg in ML-1 cells (n=9) and 64+/-23%/mg in Onco DG-1 cells (n=6) under identical incubation conditions. Preloading with tyrosine methyl ester increased cellular FET uptake dose dependently in FRTL-5 cells (165+/-25%, n=6). While TSH increased the uptake of FDG in FRTL-5 cells by sixfold, there was no TSH effect on FET accumulation. FET uptake by TSH-treated FRTL-5 cells was sodium independent and significantly inhibited by BCH (91.4+/-3.0%, n=9), tryptophan (94.8+/-1.6%, n=8) and serine (83.2+/-10.8%, n=12). TSH-starved FRTL-5 cells had a sodium-dependent component with a similar inhibition pattern. Onco DG-1 mainly confirmed the inhibition pattern of FET uptake in FRTL-5 cells, reflecting System-L-mediated FET uptake that was blocked by BCH and serine (72-85%, n=9). ML-1 cells revealed a pronounced sodium-dependent FET uptake that was inhibited by tryptophan (70+/-10%, n=9, P<.05) in the presence and in the absence of sodium, suggesting a contribution of alternative amino acid carriers.

CONCLUSION

FET uptake by FRTL-5 cells is not TSH dependent. FET uptake by FRTL-5 cells seems to be mainly mediated by a carrier exhibiting the characteristics of the System L amino acid transporter. FET uptake in thyroid cells and thyroid carcinoma cells was in the same range as that in a glioblastoma cell line. This encourages further research efforts towards the clinical evaluation of FET for the diagnostic workup of well-differentiated thyroid carcinomas.

Intracellular localization of p40, a protein identified in a preparation of lysosomal membranes

Unlike lysosomal soluble proteins, few lysosomal membrane proteins have been identified. Rat liver lysosomes were purified by centrifugation on a Nycodenz density gradient. The most hydrophobic proteins were extracted from the lysosome membrane preparation and were identified by MS. We focused our attention on a protein of approx. 40 kDa, p40, which contains seven to ten putative transmembrane domains and four lysosomal consensus sorting motifs in its sequence. Knowing that preparations of lysosomes obtained by centrifugation always contain contaminant membranes, we combined biochemical and morphological methods to analyse the subcellular localization of p40. The results of subcellular fractionation of mouse liver homogenates validate the lysosomal residence of p40. In particular, a density shift of lysosomes induced by Triton WR-1339 similarly affected the distributions of p40 and beta-galactosidase, a lysosomal marker protein. We confirmed by fluorescence microscopy on eukaryotic cells transfected with p40 or p40-GFP (green fluorescent protein) constructs that p40 is localized in lysosomes. A first molecular characterization of p40 in transfected Cos-7 cells revealed that it is an unglycosylated protein tightly associated with membranes. Taken together, our results strongly support the hypothesis that p40 is an authentic lysosomal membrane protein.

Leishmanicidal activity of amino acid and peptide esters

Amino acid and dipeptide esters kill intracellular and isolated L. amazonensis amastigotes. Several o f the compounds also restrict the growth o f mouse lesions after intralesional administration. However, the esters are known to be toxic in vitro for monocytes and certain lymphoid cells. Michel Rabinovitch surveys the mechanisms o f the leishmanicidal activity, describes some structure--activity relationships, and discusses strategies for the design of compounds more selective for the parasite.

LDL transcytosis by protein membrane diffusion

Endothelial cell (EC) cultures of different, selected vascular beds and/or organs were screened for receptor-mediated transport of proteins with a semipermeable filter assay. In SVEC4-10 cells, a mouse lymphoid endothelial cell line, orosomucoid, albumin, insulin and LDL were transcytosed from the apical (luminal) to basal (abluminal) side by a receptor-mediated pathway. Specific LDL transcytosis involved transport of intact LDL. A pathway of degradation of LDL and basal release involved vesicles in transport to lysosomes and amino acid merocrine secretion. This newly described transcellular passage of LDL via lysosomes, as well as the standard pathway, were reduced to 70% by PEG(50)-cholesterol (PEG-Chol). Combined results of temperature-dependence analysis and PEG(50)-cholesterol sensitivity show that two pathways contribute to general LDL transcellular passage. We suggest a mechanism of domain hopping by protein membrane diffusion of receptors as the pathway for intact LDL delivery. Based on theoretical considerations we propose that active transport by protein membrane diffusion can be facilitated by an organizational structure of lipid microdomains and polar cellular organization.

A family of yeast proteins mediating bidirectional vacuolar amino acid transport

Seven genes in Saccharomyces cerevisiae are predicted to code for membrane-spanning proteins (designated AVT1-7) that are related to the neuronal gamma-aminobutyric acid-glycine vesicular transporters. We have now demonstrated that four of these proteins mediate amino acid transport in vacuoles. One protein, AVT1, is required for the vacuolar uptake of large neutral amino acids including tyrosine, glutamine, asparagine, isoleucine, and leucine. Three proteins, AVT3, AVT4, and AVT6, are involved in amino acid efflux from the vacuole and, as such, are the first to be shown directly to transport compounds from the lumen of an acidic intracellular organelle. This function is consistent with the role of the vacuole in protein degradation, whereby accumulated amino acids are exported to the cytosol. Protein AVT6 is responsible for the efflux of aspartate and glutamate, an activity that would account for their exclusion from vacuoles in vivo. Transport by AVT1 and AVT6 requires ATP for function and is abolished in the presence of nigericin, indicating that the same pH gradient can drive amino acid transport in opposing directions. Efflux of tyrosine and other large neutral amino acids by the two closely related proteins, AVT3 and AVT4, is similar in terms of substrate specificity to transport system h described in mammalian lysosomes and melanosomes. These findings suggest that yeast AVT transporter function has been conserved to control amino acid flux in vacuolar-like organelles.

Lysosomal transport disorders

In the group of lysosomal storage diseases, transport disorders occupy a special place because they represent rare examples of inborn errors of metabolism caused by a defect of an intracellular membrane transporter. In particular, two disorders are caused by a proven defect in carrier-mediated transport of metabolites: cystinosis and the group of sialic acid storage disorders (SASD). The recent identification of the gene mutations for both disorders will improve patient diagnosis and shed light on new physiological mechanisms of intracellular trafficking.

Cysteine transport in melanosomes from murine melanocytes

The synthesis of pheomelanin requires the incorporation of thiol-containing compound(s) during the process of mammalian melanogenesis. Since melanins are produced only in specialized, membrane-bound organelles, known as melanosomes, such thiol donor(s) must cross the membrane barrier from the cytosol to the melanosome interior. Cysteine and/or glutathione (GSH) were proposed as suitable thiol donors, although uptake of these compounds into melanosomes was not previously characterized. In this study, we show that cysteine is transported, in a temperature- and concentration-dependent manner, across membranes of melanosomes derived from murine melanocytes. Additional proof that cysteine uptake results from a carrier-mediated process and is not due to simple diffusion or to a membrane channel, was obtained in countertransport experiments, in which melanosomes preloaded with cysteine methyl ester took up significantly more [35S]cysteine than did unloaded controls. In contrast, we were unable to detect any significant uptake of [35S]GSH over a wide concentration range, in the presence or in the absence of reducing agent. This study is the first demonstration of melanosomal membrane transport of cysteine, and it strongly suggests that free cysteine is the thiol source utilized for pheomelanin synthesis in mammalian melanocytes.

Tyrosine transport into melanosomes is increased following stimulation of melanocyte differentiation

A variety of physiological factors can stimulate differentiation of melanocytes to increase pigmentation, and critical to this process is the transport of the melanogenic substrate (tyrosine) into melanosomes. In this study, we examined whether stimulation of melanogenesis affects melanosomal tyrosine transport. Tyrosine uptake increased almost 2-fold in melanosomes derived from melanocytes treated with melanocyte-stimulating hormone (MSH), which acts to increase intracellular cAMP levels, resulting in the up-regulation of many genes involved in melanogenesis. Stimulation of melanoma cells with dibutyryl cAMP increased melanosomal tyrosine transport 2- to 3-fold after 24 to 48 hrs, with peak levels occurring after 3 to 5 days of treatment, suggesting that de novo gene expression may be required. The cAMP-induced increase in melanosomal tyrosine transport could be effectively competed with phenylalanine or tryptophan, but not with dopamine or proline, suggesting either that a pool of transporters with greater tyrosine transporting ability pre-exists, or that a greater number of tyrosine transporters reside within the melanosomal membrane. These results illustrate a rare example of hormonal plasma membrane stimulation which transduces a signal for increased vesicular transport of an amino acid.

Characterization of a melanosomal transport system in murine melanocytes mediating entry of the melanogenic substrate tyrosine

In this study, we identify a transport system for tyrosine, the initial precursor of melanin synthesis, in the melanosomes of murine melanocytes. Melanosomes preloaded with tyrosine demonstrated countertransport of 10 microM [3H]tyrosine, indicating carrier-mediated transport. Melanosomal tyrosine transport was saturable, with an apparent Km for tyrosine transport of 54 microM and a maximal velocity of 15 pmol of tyrosine/unit of hexosaminidase/min. Transport was temperature-dependent (Ea = 7.5 kcal/mol) and showed stereospecificity for the l-isomer of tyrosine. Aromatic, neutral hydrophobic compounds (such as tryptophan and phenylalanine), as well as the small, bulky neutral amino acids (such as leucine, isoleucine, and methionine) competed for tyrosine transport. Tyrosine transport was inhibited by the classical system L analogue, 2-aminobicyclo[2.2.1]heptane-2-carboxylic acid and by monoiodotyrosine, but not by cystine, lysine, glutamic acid, or 2-(methylamino)-isobutyric acid. Tyrosine transport showed no dependence on Na+ or K+, and did not require an acidic environment or the availability of free thiols. These results demonstrate the existence of a neutral amino acid carrier in murine melanocyte melanosomes which resembles the rat thyroid FRTL-5 lysosomal system h. This transport system is critical to the function of the melanosome since tyrosine is the essential substrate required for the synthesis of the pigment melanin.

Detection and characterization of a transport system mediating cysteamine entry into human fibroblast lysosomes. Specificity for aminoethylthiol and aminoethylsulfide derivatives

The uptake of [3H]cysteamine by Percoll-purified human fibroblast lysosomes was investigated to determine whether lysosomes contain a transport system recognizing cysteamine. Lysosomal cysteamine uptake is a Na(+)-independent process which rapidly attains a steady state within 1 min at pH 7.0 and 37 degrees C. A biphasic Arrhenius plot is observed for cysteamine uptake, giving a Q10 of 2.2 from 17 to 26 degrees C and a Q10 of 1.2 from 27 to 35 degrees C. The rate of lysosomal cysteamine uptake is maximal at pH 8.2, half-maximal at pH 6.8, and declines approximately 50-fold from the maximum to show very little transport at pH 5.0. Cysteamine uptake into fibroblast lysosomes displays complete saturability with a Km of 0.88 mM and Vmax of 1410 pmol of beta-N-acetylhexosaminidase/min at pH 7.0 and 37 degrees C. Analog inhibition studies demonstrated that all analogs recognized thus far by the cysteamine carrier are either aminothiols or aminosulfides and contain an amino group and sulfur atom separated by a carbon chain, 2 carbon atoms in length. The Ki constants for these analogs as competitive inhibitors of lysosomal cysteamine uptake are 2-(ethylthio)ethylamine (0.64 mM), 1-amino-2-methyl-2-propanethiol (0.74 mM), 2-dimethylaminoethanethiol (0.87 mM), thiocholine (1.6 mM), and bis(2-aminoethyl)sulfide (4.9 mM). L-Cysteine, D-penicillamine, and analogs lacking either a sulfur atom or amino group are not recognized by the cysteamine carrier including ethanolamine, choline, taurine, beta-mercaptoethanol, ethylenediamine, cadaverine, spermine, spermidine, histamine, dopamine, and 3-hydroxytyramine. In a cystine-depletion assay, a 2-h exposure of cystinotic fibroblasts to 1 mM 1-amino-2-methyl-2-propanethiol lowers cell cystine levels to the same low level obtained with cysteamine. Thus, all four aminothiols, known to deplete cystinotic fibroblasts of their accumulated cystine, are recognized as substrates by the lysosomal cysteamine carrier, suggesting the importance of this transporter in the delivery of aminothiols to the lysosomal compartment.

The thyrotropin receptor

This chapter has outlined the complex process required for thyroid growth and function. Both events are regulated by TSHR via a multiplicity of signals, with the aid of and requirement for a multiplicity of hormones that regulate the TSHR via receptor cross-talk: insulin, IGF-I, adrenergic receptors, and purinergic receptors. Cross-talk appears to regulate G-protein interactions or activities induced by TSH as well as TSHR gene expression. The TSHR structure and its mechanism of signal transduction is being rapidly unraveled in several laboratories, since the recent cloning of the receptor. In addition, the epitopes for autoantibodies against the receptor that can subvert the normal regulated synthesis and secretion of thyroid hormones, causing hyper- or hypofunction, have been defined. Studies of regulation of the TSHR minimal promotor have uncovered a better understanding of the mechanisms by which TSH regulates both growth and function of the thyroid cell. A key novel component of this phenomenon involves TSH AMP positive and negative regulation of the TSHR. Negative transcriptional regulation is a common feature of MHC class I genes in the thyroid. Subversion of negative regulation or too little negative regulation is suggested to result in autoimmune disease. Methimazole and iodide at autoregulatory levels may be important in reversing this process and returning thyroid function to normal. Their action appears to involve factors that react with the IREs on both the TSHR and the TG promoter. Too much negative regulation, as in the case of ras transformation, results in abnormal growth without function. TTF-1 is implicated as a critical autoregulatory component in both positive and negative regulation of the TSHR and appears to be the link between TSH, the TSHR, TSHR-mediated signals, TG and TPO biosynthesis, and thyroid hormone formation. Differentially regulated expression of the TSHR and TG by cAMP and insulin depend on differences in the specificity of the TTF-1 site, that is, the lack of Pax-8 interactions with the TSHR, and the IRE sites. Single-strand binding proteins will become important in determining how TSHR transcription is controlled mechanistically.

Lysosomal sulfate efflux following glycosaminoglycan degradation: measurements in enzyme-supplemented Maroteaux-Lamy syndrome fibroblasts and isolated lysosomes

Studies using lysosomal membrane vesicles have suggested that efflux of the sulfate that results from lysosomal glycosaminoglycan degradation is carrier-mediated. In this study, glycosaminoglycan degradation and sulfate efflux were examined using cultured skin fibroblasts and lysosomes deficient in the lysosomal enzyme N-acetylgalactosamine-4-sulfatase. Such fibroblasts store dermatan sulfate lysosomally, which could be labelled biosynthetically with Na2(35)SO4. The addition of recombinant N-acetylgalactosamine-4-sulfatase to the media of 35S labelled fibroblasts degraded up to 82% of the stored dermatan [35S] sulfate over a subsequent 96 h chase and released inorganic [35S] sulfate into the medium. In the presence of 4-acetamido-4'-isothiocyanatostilbene-2,2'-disulfonic acid (SITS), sulfate was reused to a minor extent in newly synthesized proteoglycan. Isolated granules from recombinant enzyme supplemented fibroblasts degraded stored dermatan [35S]sulfate to sulfate which was rapidly released into the medium at a rate that was reduced by the extra-lysosomal presence of the lysosomal sulfate transport inhibitors SITS, Na2SO4 and Na2MoO4. SITS also inhibited dermatan sulfate turnover, although it had no effect on the action of purified recombinant enzyme in vitro. These data imply that sulfate clearance occurred concomitantly with dermatan sulfate turnover in the lysosome even at high substrate loading, and that lysosome-derived sulfate, while available, is reutilized minimally in synthetic pathways.

Characteristics of taurine transport in rat liver lysosomes

Taurine (2-aminoethanesulfonic acid) is a unique sulfur amino acid derivative that has putative nutritional, osmoregulatory, and neuroregulatory roles and is highly concentrated within a variety of cells. The permeability of Percoll density gradient purified rat liver lysosomes to taurine was examined. Intralysosomal amino acid analysis showed trace levels of taurine compared to most other amino acids. Taurine uptake was Na(+)-independent, with an overshoot between 5-10 minutes. Trichloroacetic acid extraction studies and detergent lysis confirmed that free taurine accumulated in the lysosomal space. Kinetic studies revealed heterogeneous uptake with values for Km1 = 31 +/- 1.82 and Km2 greater than 198 +/- 10.2 mM. The uptake had a pH optimal of 6.5 and was stimulated by the potassium specific ionophore valinomycin. The exodus rate was fairly rapid, with a t1/2 of 5 minutes at 37 degrees C. Analog inhibition studies indicated substrate specificity similar to the plasma membrane beta-alanine carrier system, with inhibition by beta-alanine, hypotaurine, and taurine. alpha-Alanine, 2-methylaminoisobutyric acid (MeAIB), and threonine were poor inhibitors. No effects were observed with sucrose and the photoaffinity derivative of taurine NAP-taurine [N-(4-azido-2-nitrophenyl)-2-aminoethanesulfonate]. In summary, rat liver lysosomes possess a high Km system for taurine transport that is sensitive to changes in K+ gradient and perhaps valinomycin induced diffusional membrane potential. These features may enable lysosomes to adapt to changing intracellular concentrations of this osmotic regulatory substance.

Separate and shared lysosomal transport of branched and aromatic dipolar amino acids

Transport systems analogous to the T and L carriers for aromatic and bulky dipolar amino acids in plasma membranes have been characterized in the membranes of intact lysosomes isolated from human fetal skin fibroblasts. While system L appears ubiquitous in plasma membranes, system T has previously been discriminated only in the plasmalemma of human red blood cells and freshly isolated rat hepatocytes. Our findings with the lysosomal systems, provisionally designated t and l, reveal both shared and dissimilar properties with the plasma membrane systems. These properties include a lack of dependency on extralysosomal Na+, differential sensitivities to the classical system L analog, 2-aminobicyclo[2.2.1]heptane-2-carboxylic acid (BCH), and the system T analog, D-tryptophan, as well as susceptibility to thiol modification at the membrane by reactivity with N-ethylmaleimide. A transport system in lysosomes from the FRTL-5 rat thyroid cell line has been described by Bernar et al. ((1986) J. Biol. Chem. 261, 17107-17112) resembles a composite of both carrier systems reported in this work.

Characterization of a transport system for anionic amino acids in human fibroblast lysosomes

L-Aspartate and L-glutamate are transported into human fibroblast lysosomes by a single, low Km, Na(+)-independent transport system, which has been provisionally named lysosomal system d. This system resembles the Na(+)-dependent plasma membrane system chi-AG, although these differences have been observed: (1) lysosomal system d recognizes the D- as well as the L-isomers of both aspartate and glutamate, whereas only for aspartate is the D-isomer recognized by system chi-AG; (2) the anion L-homocysteate is not accepted by system chi-AG, but is an effective inhibitor of lysosomal system d; (3) N-methyl, alpha-methyl, and omega-hydroxamate derivatives of both aspartate and glutamate inhibit lysosomal system d, but only the aspartate derivatives are accepted by system chi-AG; (4) lysosomal system d shows a preference for the substrate amino group in the alpha-position, a preference not seen for system chi-AG. These points imply differences at the two recognition sites with respect to substrate length, size, and rotation, with the lysosomal site generally being the less restrictive.

Stearylamine permeabilizes the lysosomal membrane to cystine and sialic acid

Cystine efflux from isolated rat liver lysosomes was enhanced by concentrations of stearylamine that were above the critical micellar concentration. Lysosomal latency, pH, and activity of the proton-translocating ATPase were largely unaffected under controlled experimental conditions. Loss of lysosomal latency was observed at higher stearylamine to protein ratios consistent with a detergent-like mechanism of action. Partially purified cultured fibroblast lysosomes with either defective cystine or sialic acid transport lost their stored material upon exposure to stearylamine. Concentrations of stearylamine which were effective for lysosomal efflux were highly toxic for cultured fibroblasts, thus limiting its use. Under specific conditions, stearylamine apparently selectively permeabilizes the lysosomal membrane. A similar acting, but less toxic agent may be of use in the treatment of lysosomal transport disorders.

Amino acid transport systems of lysosomes: possible substitute utility of a surviving transport system for one congenitally defective or absent

Ways in which other transport systems may compensate for one that is genetically defective are considered. Comparisons of the transport systems of organelles (here the lysosome) with the transport system at the plasma membrane has significant implications for chemotherapy.