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L-lactate transport in Ehrlich ascites-tumour cells
This study examined how lactate moves in and out of Ehrlich ascites-tumour cells. Researchers used radioactive lactate and measured H+ ion movements to track transport. They found that lactate transport depends on pH and is electroneutral. The process has a high maximum rate and is temperature-sensitive. Lactate transport was inhibited by other monocarboxylic acids like pyruvate and by specific inhibitors like mersalyl. In contrast, acetate and propionate transport was not affected by these inhibitors. The study also showed that lactate influx and efflux are symmetrical. The findings suggest that lactate transport is mediated by a specific carrier that can transport substituted monocarboxylic acids but not unsubstituted ones.
Area of Science:
- Cell membrane transport mechanisms in oncology
- Monocarboxylic acid transport in tumor biology
Background:
Transport mechanisms of monocarboxylic acids in tumor cells remain partially unresolved. Prior research has shown that lactate is a key metabolite in cancer metabolism, but the specific transporters and their regulatory properties are not fully characterized. Established knowledge includes the role of lactate in tumor acid-base balance and energy metabolism. That uncertainty drove the need to investigate lactate transport in Ehrlich ascites-tumour cells. This gap motivated the study of transport kinetics and inhibitor sensitivity. No prior work had resolved the carrier specificity for substituted versus unsubstituted monocarboxylic acids. The study aimed to clarify whether lactate transport is mediated by a shared or distinct mechanism compared to other acids. This paper's contribution is to provide detailed kinetic and pharmacological evidence for a specific lactate transporter in these tumor cells.
Purpose Of The Study:
The aim of this study was to characterize lactate transport in Ehrlich ascites-tumour cells. The specific problem addressed is the mechanism and regulation of lactate movement across the cell membrane. The motivation stems from the need to understand how tumor cells manage lactate flux, which is crucial for their metabolic adaptation. The study focused on transport properties such as pH dependence, kinetic parameters, and inhibitor sensitivity. Researchers proposed to determine whether lactate transport is mediated by a specific carrier or a general monocarboxylic acid transporter. The study also aimed to compare lactate transport with that of other acids like acetate and propionate. By measuring influx and efflux rates, the researchers sought to determine if lactate transport is symmetrical. This work provides a detailed profile of lactate transport in a well-characterized tumor model.
Main Methods:
The study used Ehrlich ascites-tumour cells to measure lactate transport. Researchers tracked lactate movement using [14C]lactate and H+ ion fluxes. Transport was assessed under varying pH conditions to determine electroneutrality. Kinetic experiments were conducted to measure apparent Km and Vmax values. The temperature dependence of transport was quantified using activation energy calculations. Competitive inhibition was tested with substituted monocarboxylic acids like pyruvate. Non-transportable analogues such as alpha-cyano-4-hydroxycinnamate were used as inhibitors. Mersalyl, a thiol-group reagent, was also tested for its inhibitory effects. These methods allowed the researchers to distinguish between carrier-mediated and passive transport mechanisms.
Main Results:
Lactate transport was found to be pH-dependent and electroneutral. The apparent Km for lactate was approximately 4.68 mM at pH 6.2 and 37°C. The maximum transport rate (Vmax) reached 680 nmol/min per mg of protein. Transport exhibited a high temperature dependence with an activation energy of 139 kJ/mol. Pyruvate was a competitive inhibitor with a Ki of 6.3 mM. Non-transportable analogues like alpha-cyano-4-hydroxycinnamate inhibited lactate transport with Ki values of 0.5 to 3.6 mM. Mersalyl also inhibited transport with a Ki of 125 μM. In contrast, acetate and propionate transport was insensitive to these inhibitors. Mersalyl was used as an inhibitor stop to measure net influx and efflux rates of lactate. Influx and efflux showed similar concentration dependence, suggesting symmetrical transport.
Conclusions:
The authors concluded that lactate transport in Ehrlich ascites-tumour cells is mediated by a specific carrier. This carrier is capable of transporting substituted monocarboxylic acids like pyruvate. The transport mechanism is distinct from that of unsubstituted short-chain aliphatic acids. The observed electroneutrality and pH dependence support a 1:1 symport with H+ ions. The high temperature dependence suggests a protein-mediated process. Competitive inhibition by non-transportable analogues indicates structural specificity. The symmetrical nature of lactate influx and efflux implies a bidirectional transporter. The findings suggest that lactate transport is a regulated process in these tumor cells.
Frequently Asked Questions
The authors propose that lactate transport is mediated by a carrier that can also transport substituted monocarboxylic acids.
The apparent Km was approximately 4.68 mM at pH 6.2 and 37°C.
Mersalyl was used as an inhibitor stop to measure initial rates of lactate influx and efflux.
Pyruvate competitively inhibits lactate transport with a Ki of 6.3 mM.
The activation energy was 139 kJ/mol, indicating a strong temperature dependence.
The authors suggest that acetate and propionate use a different transport mechanism, as they are insensitive to lactate transport inhibitors.