The mechanism of variation inlysine and threonine conservation was investigated by measuring the activity of the enzyme initiating the amino acid catabolism in adult rats adapted to different diets. Both liver threonine dehydratase and lysine-ketoglutarate reductase activities were increased by high protein intake. Supplementation with 2% L-lysine HCI to a 5% lactalbumin diet increased liver lysine-ketoglutarate reductase activity three-fold. Feeding a lysine-free diet or a 10% wheat gluten diet significantly decreased the activity below the level obtained by feeding a protein-free diet. In contrast, dietary threonine in either excess or deficiency did not change the liver threonine dehydratase activity. At the same level of lysine intake, an excess of dietary wheat gluten seemed to increase the liver lysine-ketoglutarate reductase activity much more than dietary lactalbumin. These data suggest that the activity of lysine-ketoglutarate reductase in rat liver might be influenced by dietary lysine as well as excesses of other amino acids in wheat gluten. The different response of these two catabolic enzymes to their substrate might explain the variation of lysine and threonine conservation during amino acid deficiency.

The addition of human platelet-derived growth factor (PDGF) to confluent, quiescent cultures of human diploid fibroblasts induced the rapid breakdown of cellular polyphosphoinositides. The levels of 32P-labeled phosphatidylinositol 4,5-bisphosphate (PIP2), phosphatidylinositol 4-phosphate (PIP), and phosphatidylinositol (PI) decreased by 30 to 40% within 1 min after exposure of the cells to PDGF. The levels of PIP and PIP2 returned to their initial values within 3 and 10 min, respectively, after PDGF addition. The level of PI continued to increase after it had returned to control values and was up threefold within 30 min after PDGF addition. In cells prelabeled with myo-[3H]inositol PDGF caused an eightfold increase in the levels of inositol trisphosphate (IP3) within 2 min. Lesser increases, twofold and 1.3-fold, respectively, were seen in levels of inositol bisphosphate (IP2) and inositol monophosphate (IP). Within 10 min after PDGF addition the levels of all three inositol phosphates had decreased to control values. The levels of IP3 measured 2 min after PDGF addition depended on the PDGF concentration and were maximal at 5-10 ng/ml of PDGF. Similar concentrations of PDGF stimulate maximal cell growth and DNA synthesis in these cells.

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The responsiveness of enterocytes to Escherichia coli heat-labile enterotoxin (LT) was studied in the small intestine of 6-to 7-week-old rats. Dose-effect analysis showed the dose required for a 50% maximal LT-induced secretory response to be at 8 nM. After the well-documented glycolipid GM1 receptor was blocked with the cholera toxin B subunit, LT still activated the second messenger cascade, measured in terms of heightened cellular adenylate cyclase activity, and caused fluid to be secreted into ligated intestinal loops. Furthermore, Scatchard analysis of binding kinetics suggested that LT bound to two receptor sites on the intestinal microvillus membrane. The toxin also bound to delipidated membrane but was competitively inhibited by a galactose-specific lectin, RCA60, suggesting that the additional receptor is a galactoglycoprotein. Western blot analysis of toxin binding to membrane proteins revealed a group of binding components around 85 to 150 kilodaltons. When measured at 2.2 nM LT, approximately 70% of LT-binding activity took place through a (ED50) was estimated by using a dose-effect analysis (2) program (Elsevier-Biosoft, Cambridge, United Kingdom) on an IBM personal computer. In blocking the toxin-induced fluid secretion, a 100-fold excess of CT-B was used. The CT B subunit was introduced into the appropriate loops 3 to 5 min before the injection of LT.Adenylate cyclase assay. Intestinal mucosa was scraped from each loop and homogenized in 0.