Lysosomes constitute only 4% of the intracellular volume of a normal human fibroblast. When human fibroblasts are incubated for 2-5 min with 20 microM [35S]cystine in Krebs-Ringer phosphate solution at pH 7.4, a minimum of 50-60% of the total radioactivity taken up by the cells is found sequestered into the lysosomal compartment in the form of cysteine. A lysosomal transport system, highly specific for cysteine, appears to facilitate this rapid lysosomal cysteine sequestration. Time courses of [35S]cysteine uptake into isolated, Percoll-purified fibroblast lysosomes at pH 7.0 and 37 degrees C are linear for the first 4-5 min and attain a steady state by 10 min. Lysosomal cysteine uptake displays a Km of 0.05 mM at pH 7.0 and an activation energy of 21 kcal/mol, corresponding to a Q10 of 3.2. The role of this transport system in delivering cysteine into lysosomes is supported by its pH curve showing a slow rate of cysteine transport at the acidic pHs between 5 and 6, but then increasing sevenfold between pH 6 and 7.5 to be maximally active near the cytosolic pH of 7. Carrier mediation by this lysosomal transport route demonstrates a high specificity for cysteine as indicated by the inability of the following amino acids to significantly inhibit at 5 mM the lysosomal uptake of 0.035 mM [35S]L-cysteine: ala, ser, pro, val, gly, homocysteine, D- or L-penicillamine, arg, asp, or leu. Similarly, D-cysteine and beta-mercaptopropionate were poor inhibitors, suggesting that both the L-isomer and alpha-amino group of cysteine appear to be required for recognition by the cysteine-specific transport system. In contrast, cysteamine, which lacks an alpha-carboxyl group, was able to strongly inhibit lysosomal cysteine uptake. The physiological importance of this cysteine-specific lysosomal transport system may be to aid lysosomal proteolysis by delivering cysteine into the lysosomal compartment to (a) maintain the catalytic activity of the thiol-dependent lysosomal enzymes and (b) break protein disulfide bridges at susceptible linkages, thereby allowing proteins to unfold, facilitating their degradation.
Skip Nav Destination
Article navigation
1 February 1990
Article|
February 01 1990
A cysteine-specific lysosomal transport system provides a major route for the delivery of thiol to human fibroblast lysosomes: possible role in supporting lysosomal proteolysis.
R L Pisoni,
R L Pisoni
Department of Pediatrics and Communicable Diseases, University of Michigan Medical School, Ann Arbor 48109-2029.
Search for other works by this author on:
T L Acker,
T L Acker
Department of Pediatrics and Communicable Diseases, University of Michigan Medical School, Ann Arbor 48109-2029.
Search for other works by this author on:
K M Lisowski,
K M Lisowski
Department of Pediatrics and Communicable Diseases, University of Michigan Medical School, Ann Arbor 48109-2029.
Search for other works by this author on:
R M Lemons,
R M Lemons
Department of Pediatrics and Communicable Diseases, University of Michigan Medical School, Ann Arbor 48109-2029.
Search for other works by this author on:
J G Thoene
J G Thoene
Department of Pediatrics and Communicable Diseases, University of Michigan Medical School, Ann Arbor 48109-2029.
Search for other works by this author on:
R L Pisoni
Department of Pediatrics and Communicable Diseases, University of Michigan Medical School, Ann Arbor 48109-2029.
T L Acker
Department of Pediatrics and Communicable Diseases, University of Michigan Medical School, Ann Arbor 48109-2029.
K M Lisowski
Department of Pediatrics and Communicable Diseases, University of Michigan Medical School, Ann Arbor 48109-2029.
R M Lemons
Department of Pediatrics and Communicable Diseases, University of Michigan Medical School, Ann Arbor 48109-2029.
J G Thoene
Department of Pediatrics and Communicable Diseases, University of Michigan Medical School, Ann Arbor 48109-2029.
Online ISSN: 1540-8140
Print ISSN: 0021-9525
J Cell Biol (1990) 110 (2): 327–335.
Citation
R L Pisoni, T L Acker, K M Lisowski, R M Lemons, J G Thoene; A cysteine-specific lysosomal transport system provides a major route for the delivery of thiol to human fibroblast lysosomes: possible role in supporting lysosomal proteolysis.. J Cell Biol 1 February 1990; 110 (2): 327–335. doi: https://doi.org/10.1083/jcb.110.2.327
Download citation file:
Sign in
Don't already have an account? Register
Client Account
You could not be signed in. Please check your email address / username and password and try again.
Could not validate captcha. Please try again.
Sign in via your Institution
Sign in via your InstitutionSuggested Content
Email alerts
Advertisement
Advertisement