Plasma Membrane Carrier Research Articles

Active-site-directed inhibition of the plasma-membrane carrier transporting short-chain, neutral amino acids into Trypanosoma brucei.

1. Glycine chloromethyl inhibited the active-transport of L-serine into bloodstream forms of Trypanosoma brucei. 2. Substrates of the short-chain, neutral amino acid transport system (N1), but not of other amino acid transport systems, protected the carrier protein from inhibition. 3. Inhibition was never more than 80% complete. The residual activity might have due to a proportion of N1 carrier active sites which had not reacted with the inhibitor. 4. The inhibition was highly selective for the N1 amino acid transport system. Other amino acid transport systems were not affected and the rate of respiration was only slightly affected. 5. The inhibition was first-order with respect to concentration, indicating that one molecule of the inhibitor reacted with each carrier active-site. 6. The high selectivity of this inhibitor should make it a useful labelling agent during the isolation and purification of the N1 amino acid transport carrier protein(s).

Factors affecting lipolysis rates in rat adipose tissue

An attempt has been made to elucidate the mechanism in rat adipose tissue by which glucose overcomes the antilipolytic action of insulin and enhances lipolysis induced by epinephrine. The possible role in this phenomenon of the insulin-activated cell membrane carrier system for glucose has been examined by studying the action of other transportable sugars. Fructose behaves like glucose but higher concentrations are needed for maximum effects. No action is shown by 3- O-methyl glucose and, 2-deoxyglucose displays antilipolytic actions of its own. An antilipolytic action of insulin is seen when lipolysis is induced by theophylline, but not when dibutyryl cyclic AMP is employed. In both cases, however, an enhanced rate of lipolysis is observed if glucose and insulin are present. In each of these situations the accumulation of free fatty acids in the tissue is high and remains unchanged in the presence of glucose. Thus the enhancement of lipolysis by glucose would not appear to be dependent solely upon its ability to furnish glycerophosphate for reesterification of free fatty acids. Involvement of glycerophosphate in some manner is suggested, however, by the fact that lactate, inactive by itself, is able to significantly augment the stimulatory action of glucose upon lipolysis. A variety of substances including glycerophosphate, which could be produced by the metabolism of glucose or result from the fatty acid reesterification process were tested for their effects upon lipolysis without success.