DIGESTIVE CAPACITY OF PURIFIED LYSOSOMES

LYSOSOMAL ENZYMES OF RAT INTESTINAL MUCOSA

Six intracellular hydrolases known to be associated with lysosomes in rat liver were found in rat intestinal mucosa. The extent to which they were particulate-bound and the degree of enzyme release when the particulate fractions were suspended in hypotonic media followed the same pattern in both mucosa and liver. The specific activities of the mucosa enzymes were either comparable to or slightly smaller than those of the liver enzymes. These results suggest that the mucosa hydrolases belong to lysosome-like particles. However, differential fractionation of the mucosa indicated that the particles from the mucosa sediment at lower centrifugal forces than do those from the liver and are more heterogeneous in size, bearing a closer resemblance to kidney lysosomes. Possible physiological functions of particulate-bound digestive enzymes in intestinal mucosa are discussed.

Manifestations of cell damage after freezing and thawing

The nature of the primary lesions suffered by cells during freezing and thawing is unclear, although the plasma membrane is often considered the primary site for freezing injury. This study was designed to investigate the nature of damage immediately after thawing, by monitoring several functional tests of the cell and the plasma membrane. Hamster fibroblasts, human lymphocytes, and human granulocytes were subjected to a graded freeze-thaw stress in the absence of cryoprotective compound by cooling at -1 degree C/min to a temperature between -10 and -40 degrees C, and then were either warmed directly in water at 37 degrees C or cooled rapidly to -196 degrees C before rapid warming. Mitochondrial function in the cells was then assessed using 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyl-tetrazolium bromide (MTT), fluorescein diacetate (FDA), colony growth, and osmometric response in a hypertonic solution. Cells behaved as osmometers after cooling at -1 degree C/min to low temperatures at which there were no responses measured by other assays, indicating that the plasma membrane is not a primary site for injury sustained during slow cooling. These results also indicate that the FDA test does not measure membrane integrity, but reflects the permeability of the channels through which fluorescein leaves the cells. Fewer cells could respond osmotically after cooling under conditions where intracellular freezing was likely, implying that the plasma membrane is directly damaged by the conditions leading to intracellular freezing. A general model of freezing injury to nucleated mammalian cells is proposed in which disruption of the lysosomes constitutes the primary lesion in cells cooled under conditions where the cells are dehydrated at low temperatures.

Comparison of folylpolyglutamate hydrolases of mouse liver, kidney, muscle and brain

The folylpolyglutamate hydrolase activities of mouse liver, kidney, muscle and brain were examined by incorporation of methylenetetrahydrofolate polyglutamate reaction products into a stable ternary complex with tritiated fluorodeoxyuridylate and L. casei thymidylate synthetase. Complexes were separated electrophoretically on the basis of charge associated with the polyglutamyl moieties to determine distribution of chain lengths throughout the time course of the reaction. Tissue folylpolyglutamate hydrolase activities were allowed to utilize endogenous folylpolyglutamate as substrates by incubating crude tissue extracts at pH 7.4 ang pH 4.5. Kidney and muscle contained relatively reactive hydrolases which were capable of generating intermediates of essentially all chain lengths from folylpentaglutamate, the predominant endogenous species. The relatively low activity in brain also gave rise to all possible intermediates. Liver contained a high concentration of methylenetetrahydrofolate but little hydrolase activity. The activity present in liver gave rise to essentially no intermediates but yielded only the monoglutamate form of the cofactor. When purified lysosomal preparations from liver and kidney were allowed to react with synthetic folylpolyglutamates, the same specificity with regard to reaction products was observed as with endogenous substrates.

Proteolysis of isolated mitochondria by myocardial lysosomal enzymes

1. Solubilized mitochondria and lysosomal fractions were obtained from guinea-pig heart by differential centrifugation and selective membrane disruption. 2. Mitochondria incubated at 37 degrees C in the presence of lysosomal enzymes underwent proteolysis. The rate of protein degradation was inversely dependent on pH. 3. The use of proteinase inhibitors showed that at low pH the major enzyme involved in mitochondrial digestion was cathepsin D. 4. At neutral pH carboxyl proteinases were still active, but thiol proteinases accounted for most of the protein breakdown. 5. The role of lysosomal enzymes as mediators of mitochondrial damage in ischaemic myocardium is discussed.

Glucagon-stimulated phosphorylation of mitochondrial and lysosomal membranes of rat liver in vivo

The pancreatic hormone, glucagon, stimulates the net uptake of inorganic (32)P in vivo into rat liver and its incorporation into proteins of microsomes, mitochondrial membranes, and lysosomes. Incorporation into cytosolic proteins was enhanced only slightly by glucagon. More than 95% of the protein-bound phosphate is present as phosphoserine. Both the radioactivity and the total amount of protein-bound phosphate are increased after injection of glucagon. Glucagon treatment enhanced (32)P incorporation into the alcohol-ether soluble lipids of mitochondria but did not alter the relative distribution of (32)P in various phospholipid species.

The localization of proteolytic activity in rat liver mitochondria and its relation to mitochondrial swelling and aging

1. On storage of rat liver mitochondria at 0 degrees , water content, total amino acid content and leakage of protein all rose steadily over a 72hr. period. The initial ratio of intramitochondrial to extramitochondrial amino acid concentration lay between 18 and 24. Initially this rose, but it then fell to 1.9 at the end of storage. The concentration gradient between internal and external amino acids was relatively constant throughout the period. These processes were accentuated at 22 degrees and 40 degrees , the concentration gradient reaching 70mumoles/ml., water content rising to 8.3mg./mg. dry wt. and protein leakage reaching 42% of total mitochondrial protein. ;Swelling agents' produced no correlated changes in amino acid production and swelling. 2. Added glutamate was not concentrated within the pellet of whole or disrupted mitochondria. Endogenous amino acids were distributed evenly between the pellet and the supernatant of disrupted mitochondria. It is concluded that amino acids are produced within mitochondria and that adsorption and uptake from the medium do not contribute significantly to amino acids in the pellet. 3. beta-Glycerophosphate, a lysosome protectant, increased amino acid production by rat liver mitochondria. Treatment with Triton X-100 and disruption by freezing and thawing showed that 56% of proteolytic activity was ;free' in whole mitochondria, whereas only 11% of acid phosphatase activity, a lysosomal enzyme, was ;free'. 4. ;Light' mitochondria contained 30% more neutral proteolytic activity but 300% more acid phosphatase activity than ;heavy' mitochondria. 5. Electron micrographs of mitochondrial preparations showed less than one particle in 500 that could be identified as a lysosome. Treatment with Triton X-100 disrupted the structure of roughly 50% of the mitochondria; the rest appeared to retain their membrane, cristae and ground substance. Freezing and thawing caused gross swelling and loss of ground substance and rupture of external membranes. 6. Of the recovered proteolytic activity, 81% at pH7.4 and 70% at pH5.8 were found in the high-speed supernatant of broken mitochondria. A further fivefold increase in specific activity was found in the first protein fraction obtained by Sephadex G-50 gel filtration. 7. Between 60 and 80% of proteolytic activity was found in the 40-60%-saturated ammonium sulphate precipitate. Almost all of the soluble-fraction proteolytic activity could be recovered in a pH5.0 supernatant. 8. The results give no support to the view that mitochondrial neutral proteolytic activity reflects lysosomal content. 9. The possible role of intramitochondrial amino acid production and the proteolysis of internal barriers in passive swelling of mitochondria is discussed.