Rapid cation flux from Torpedo californica membrane vesicles: comparison of acetylcholine receptor enriched and selectively extracted preparations.

Rapid efflux of ²²Na from within closed vesicles derived from $\text{Torpedo}\text{californica}$ electroplax membranes has been studied as an $\text{in}\text{vitro}$ assay of acetylcholine receptor functionality. The most highly purified membrane preparations contained major polypeptides of M.W. 43 and 90 × 10³ daltons in addition to the four peptides characteristic of the acetylcholine receptor (40, 50, 60, 65 × 10³ daltons). Removal of these extra peptides by base extraction did not significantly alter the characteristics of carbamylcholine induced ²²Na efflux: the agonist dose response curve was similar, preequilibration with agonist caused desensitization, the irreversible antagonist α-Bungarotoxin blocked the efflux and the reversible blockade by the neurotoxin perhydrohistrionicotoxin was also retained. The dose response curve for perhydrohistrionicotoxin corresponded closely to its known binding characteristics for base extracted membranes.     

Binding of polycyclic aromatic hydrocarbons to DNA of cells in culture: a rapid method for its analysis using hydroxylapatite column chromatography.

A rapid procedure to study the interaction of carcinogens with DNA in cultured cells has been developed. The cells, which are labeled with 7,12-[3H]dimethylbenz[a] anthracene ([3H]DMBA), are lysed with 0.24 M phosphate buffer (pH 6.8), 1% sodium dodecyl sulfate (SDS), 8 M urea and 0.01 M ethylenediamine-tetraacetate (EDTA) and sonicated. The cell lysates are fractionated on columns of hydroxylapatite. Proteins and RNA are removed with 8 M urea in 0.24 M phosphate buffer (pH 6.8). DMBA-bound DNA is eluted with 0.4 M phosphate buffer (pH 6.8). DMBA-DNA isolated by this procedure is virtually free from proteins and RNA. Thermal stability, ultraviolet spectra and the density of DNA is not altered by DMBA binding. The uptake of DMBA by mouse epidermal cells is rapid and the binding of DMBA to DNA is linear for the first 8 h of exposure. DMBA binds to DNA in all phases of the cell cycle. However, the highest binding occurs immediately following maximum DNA synthesis.     

The preparation of lipid-depleted bacteriorhodopsin

Bacteriorhodopsin, the protein of the purple membrane of Halobacterium halobium, was freed to the extent of 90–95% from the natural membrane lipids without loss of function. The residual lipid corresponded to less than 1 mol/mol of bacteriorhodopsin. Delipidation was achieved by treatment of the purple membrane with a mixture of the detergent dimethyldodecylamine oxide and sodium chloride. The detergent was removed by dialysis or by sucrose density gradient centrifugation. Analysis of the lipids removed and those still bound to bacteriorhodopsin was facilitated by the use of purple membrane preparations labelled with ³⁵S, ³²P, or ¹⁴C. The composition of the residual lipids associated with bacteriorhodopsin was similar to that of the total lipid in the purple membrane.     

Partial sequence of immunoglobulin light-chain isolated from purified plasma membranes of mouse myeloma cells.

Plasma membranes were prepared from MOPC-321 mouse myeloma cells incubated with [³H]leucine. The L-chain from the purified plasma membranes was isolated, it was subjected to radioactive sequence analysis, and leucine was found at positions 4, 11 and 15. This sequence matches with that of the mature L-chain (Leu at positions 4, 11, 15, etc.) and differs from that of the L-chain precursor that contains a hydrophobic N-terminal extra piece (Leu at positions 6, 7, 8, 11, 12, 13, 24, 31, 35, etc.). This result establishes mature L-chain in the surface membrane of plasmacytoma cells.