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microsomes via a trans-membrane transporter protein. Once inside the vesicle, glucose
6-phosphate is cleaved yielding glucose and free phosphate. It was noted that if the
membrane fractions were bathed in Pb 2 รพ , insoluble lead phosphate (Pb 3 (PO 4 ) 2 )
accumulated in the ER microsome interior, significantly increasing their density. The ER
microsomes could then be easily centrifuged away from the other microsomes.
E. MEMBRANE MARKERS
For each of the many steps in isolating and purifying a membrane, tests must be run not
only to demonstrate that the target membrane is indeed present and being purified but also
that possible contaminating membranes are being eliminated. For the initial run through
a multi-step procedure this may require a tremendous amount of very tedious assays. Posi-
tive identification of a membrane is achieved by following two basic types of membrane
markers, structural markers and marker enzymes [49] . Both types of markers must uniquely
reside in the membrane being tested. To follow the outcome of each membrane purification
step, relatively rapid and quantitative procedures, mostly enzyme assays, are preferred
over slower and more qualitative procedures like electron microscopy. After the investi-
gator has achieved what he hopes is high purity, a variety of confirming enzyme assays,
structural measurements, and compositional analysis are preformed. Once the membrane
purification scheme is shown to be accurate and reproducible, subsequent purifications
need not be so detailed. Some of these markers are listed in Tables 12.2 (Membrane Struc-
tural Markers) and Table 12.3 (Membrane Marker Enzymes) (these tables are modified
from Table 1.2 in [49] .
TABLE 12.2 Partial list of Membrane Structural Markers.
Membrane
Structural marker
Nucleus
Chromosomes
DNA (Feulgen Stain)
Plasma Membrane
Gap Junctions
Brush Border Glycocalyx
Microvilli
Bind Lectins
Mitochondrial Inner
F 1 -ATPase 'knob'
Cardiolipin
RER
Ribosomes
Golgi
Stains well with Osmium
Thylakoid
Chlorophyll
Any Membrane
Specific Antibodies
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