Two plasma proteins, vitamin D-binding protein (actin monomer sequestrant) and gelsolin (actin polymer severing), have been found in association with actin in plasma from ill humans and during experimental injury. In vitro, these are the only plasma proteins that display a high affinity for actin. We infused increasing amounts of globular actin intravenously to rats to evaluate its disposition in plasma and tissues. Intravascular filament formation, microthrombi, and endothelial injury were observed, especially in the pulmonary circulation. These pathological changes were not observed when the globular actin in the infusate had been preincubated with the vitamin D-binding protein in vitro. Complexes of actin with both proteins were found in the plasma, suggesting a saturable, plasma actin-binding system in vivo. Our findings suggest that in vivo saturation of these proteins' actin-binding capacities may serve as a paradigm for pulmonary vascular disorders seen during widespread tissue trauma and cell lysis.

Vitamin D-binding protein (DBP) is an abundant plasma protein. The observation of immunodetectable, cell-associated DBP on peripheral blood mononuclear cells and placental cytotrophoblasts had presented the question of the origin, function, and precise subcellular localization of cell-associated DBP. Using anti-human DBP F(ab')2 with fluorescence-activated cytometric analysis and immunogold electron microscopy, we detected DBP on the plasmalemma of U937 cells, a monoblastic, histiocytic cell line grown in media supplemented with fetal calf serum (FCS). DBP was then removed from FCS by actin affinity chromatography followed by anti-DBP immunoaffinity chromatography. U937 cells in this DBP-free medium exhibited nearly identical growth rates to cells grown in medium containing native FCS. However, in contrast to cells grown with native FCS, those grown for seven to eight generations with DBP-free FCS exhibited less cell-surface DBP as quantified by fluorescence-activated cytometric analysis (73% decrease) and immunoelectron microscopy (88% decrease). DBP mRNA could not be detected in U937 cells, placental tissues, freshly prepared resting and stimulated B and T lymphocytes, or lymphocyte-derived cell lines by Northern analysis. In addition, using the sensitive reverse transcriptase/polymerase chain reaction assay no DBP fragments were detectable in U937 cells. We conclude that U937 cell-associated DBP is exogenously derived from plasma and is located on the plasmalemma. Based upon this conclusion, we postulate that specific binding sites for DBP may exist on the plasma membranes of certain cell types.

SUMMARYThe effect of gut glucagon-like immunoreactivity (GLI) devoid of pancreatic glucagon was studied in piglets. All glucagon-like peptides with an accessible C-terminal were removed from the gut extract by specific antibodies reacting with the C-terminal of the glucagon molecule. Endogenous secretion of pancreatic and gut glucagon was blocked by somatostatin infusion, and then the purified gut glucagon preparation was infused. The latter prevented the hypoglycemia resulting from somatostatin infusion, and increased the glucagon level detectable by C-terminal specific antibodies in the blood of the animals. This rise was significant statistically from the 30th min of GLI administration (26.7 ± 7.2 pg/ml versus 137.0 ± 67.0 pg/ml; P < 0.05) and increased until the end of the infusion (90th min, 218 ± 60 pg/ml; P < 0.005). It has been suggested that, owing to the in vivo enzymatic degradation of the infused gut glucagon, biologically active "pancreatic" glucagon fractions are formed extracellularly. DIABETES 30:792-794, September 1981. R ecently data have been accumulated pointing to the similarities of pancreatic glucagon derived from the A-cells of the pancreas and the stomach, and enteric glucagon-like immunoreactivity (GLI) produced by the L-cells of the gut, and raising the possibility of a common biosynthetic precursor. One of the components of gut GLI, the 100-amino acid polypeptide, glicentin, has been shown to contain the full sequence of glucagon, extended from the C-terminus by an additional 8-amino acid chain and from the N-terminus by a 63 amino-acid chain.

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