INTRODUCTIONThe membrane-attached polysomes of the liver cells are assumed to be the intracellular site of albumin synthesis (for recent critical review see Campbell (1)) . The histological localization of albumin by fluorescent antisera has been described repeatedly (3, 5-7) . It is not possible, however, to decide whether albumin localized in the cell by this technique is newly synthesized, or taken up by the cells from the blood, or simply adsorbed during preparation .In radioautographs we observed predominant labeling of the periportal area of the liver lobules after intraportal injection of leucine-3H, whereas intracaval injection led to uniform labeling throughout the lobule . The way of administration of the tracer thus permits a study of protein synthesis of selected regions of the liver lobule .Recently, the isolation of radiochemically pure albumin from liver homogenates and a method to determine the proportion of albumin to total protein synthesis have been described (9) .In this report the ratio of albumin to total protein synthesis after intraportal injection was compared to that after intracaval injection . By combining these results with the radioautographic observations, it was possible to decide whether or not albumin synthesis was distributed uniformly throughout the liver lobule .
MATERIALS AND METHODSAnimals and biochemical methods have been described previously (8, 9) .For radioautography small pieces of liver were fixed in 0.75 M sodium phosphate buffer of pH 7 .0 with 4% formaldehyde for 24 hr at 4°C, dehydrated in ethanol, and embedded in paraffin blocks . Slices of 5 µ and control slices from unlabeled livers were cut and transferred to the same microscopical slides . Paraffin was removed by alcohol, methylbenzoate, and xylene. The slices were washed twice for 20 min in 0 .76 mm nonradioactive leucine and then dipped into Ilford K5 liquid emulsion (Ilford Ltd ., Ilford, Essex, England) . After 24 days' exposure at 18°C and after photographic development, radioautographs were stained through the emulsion with Mayer's hemalum, made transparent by 70% ethanol with 4 vol % of 1 N HCI, rinsed in running tap water, and dried .The density of silver grains was determined by leading a measuring diaphragm of 552 µ2 around the central and the periportal areas of the liver THE JOURNAL OF CELL BIOLOGY • VOLUME 47, 1970 • pages 285-289 285 on May 13, 2018 jcb.rupress.org Downloaded from http://doi.org/10.1083/jcb.47.1.285 Published Online: 1 October, 1970 | Supp Info:lobule and measuring the light reflected by the silver grains in a microscope equipped with a photomultiplier (4) . The pulses of the photomultiplier were recorded as "working units" by a Knott analog digital counter . Two animals were measured after intraportal, and two after intracaval injection of the label . Almost identical values were obtained within each group . The obtained values were averaged after subtraction of the background, which was determined with unlabeled slices . After intraportal injection, 100 positions of the diaph...
