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Biomedical subjects

M A Block

Publications and source records attributed to M A Block.

At least 37 records · Page 2Linked to original sources
PubMed1985

Localization and synthesis of prenylquinones in isolated outer and inner envelope membranes from spinach chloroplasts.

The prenylquinone content and biosynthetic capabilities of membrane fractions enriched in outer and inner envelope membranes from spinach chloroplasts were analyzed. Both envelope membranes contain prenylquinones, and in almost similar amounts (on a protein basis). However, the outer envelope membrane contains more alpha-tocopherol than the inner one although this prenylquinone is the major one in both fractions. On the contrary, plastoquinone-9 is present in higher amounts in the inner envelope membrane than in the outer one. In addition, it has been demonstrated that all the enzymes involved in the last steps of alpha-tocopherol and plastoquinone-9 biosynthesis, i.e., homogentisate decarboxylase polyprenyltransferase, S-adenosyl-methionine:methyl-6-phytylquinol methyltransferase, S-adenosyl-methionine: alpha-tocopherol methyltransferase, homogentisate decarboxylase solanesyltransferase, S-adenosyl-methionine:methyl-6-solanesylquinol methyltransferase, and possibly 2,3-dimethylphytylquinol cyclase, are localized on the inner envelope membrane. These results demonstrate that the inner membrane of the chloroplast envelope plays a key role in chloroplast biogenesis, and especially for the synthesis of the two major plastid prenylquinones.

Cell Fractionation↗
PubMed1985

Localization of phosphatidylcholine in outer envelope membrane of spinach chloroplasts.

We have examined the effects of phospholipase C from Bacillus cereus on the extent of phospholipid hydrolysis in envelope membrane vesicles and in intact chloroplasts. When isolated envelope vesicles were incubated in presence of phospholipase C, phosphatidylcholine and phosphatidylglycerol, but not phosphatidylinositol, were totally converted into diacylglycerol if they were available to the enzyme (i.e., when the vesicles were sonicated in presence of phospholipase C). These experiments demonstrate that phospholipase C can be used to probe the availability of phosphatidylcholine and phosphatidylglycerol in the cytosolic leaflet of the outer envelope membrane from spinach chloroplasts. When isolated, purified, intact chloroplasts were incubated with low amounts of phospholipase C (0.3 U/mg chlorophyll) under very mild conditions (12 degrees C for 1 min), greater than 80% of phosphatidylcholine molecules and almost none of phosphatidylglycerol molecules were hydrolyzed. Since we have also demonstrated, by using several different methods (phase-contrast and electron microscopy, immunochemical and electrophoretic analyses) that isolated spinach chloroplasts, and especially their outer envelope membrane, remained intact after mild treatment with phospholipase C, we can conclude that there is a marked asymmetric distribution of phospholipids across the outer envelope membrane of spinach chloroplasts. Phosphatidylcholine, the major polar lipid of the outer envelope membrane, is almost entirely accessible from the cytosolic side of the membrane and therefore is probably localized in the outer leaflet of the outer envelope bilayer. On the contrary, phosphatidylglycerol, the major polar lipid in the inner envelope membrane and the thylakoids, is probably not accessible to phospholipase C from the cytosol and therefore is probably localized mostly in the inner leaflet of the outer envelope membrane and in the other chloroplast membranes.

Chloroplasts↗
PubMed1985

Localization of residual medullary thyroid cancer by thallium/technetium scintigraphy.

Whole-body thallium/technetium subtraction scintiscans, with thallium 201 chloride and technetium 99m pertechnetate, were obtained on 10 patients with medullary thyroid cancer with postoperative elevations of serum calcitonin values. Seven patients had the hereditary variant of medullary thyroid cancer (the multiple endocrine neoplasia, type II syndrome) while three patients had the sporadic form. Negative scans were obtained on five patients with basal calcitonin levels less than 3 ng/ml (normal less than 0.35 ng/ml). Five other patients with elevated calcitonin levels (range 3.6 to 69.2 ng/ml; mean 28.3 ng/ml) had positive scans that have guided further surgical approaches. Serum calcitonin appears to remain the most sensitive indicator of residual medullary thyroid cancer while localization of this residual tumor tissue frequently can be obtained through thallium/technetium subtraction imaging in both the hereditary and sporadic forms of the disease.

Adolescent↗
PubMed1984

Clinical value of tumor doubling estimations in multiple endocrine neoplasia type II.

Experience with children with multiple endocrine neoplasia (MEN) type IIb has emphasized that medullary thyroid cancer (MTC) in MEN IIb is more aggressive than in MEN IIa. Earlier ages of onset and apparently more rapid growth of MTC in MEN IIb suggest that these tumors have earlier ages of conversion to malignant states and/or shorter doubling times. The age at which a hyperplastic C cell becomes a malignant cell and the true doubling time cannot be estimated presently. Maximum volume-doubling times of 35 and 75 days (21 to 26 doublings) were calculated from tumor size and age at operation in five patients with MEN IIb aged 2 to 5 years. Calculations in 20 patients with MEN IIa revealed maximum doubling times of 110 to 440 days, with ages ranging from 7 to 29 years and number of doubling ranging from 18 to 38. Positive provocative calcitonin tests in two adult patients with MEN IIa after 10 to 11 years of repeated negative tests suggest a minimum doubling time of 190 to 210 days. Such experience emphasizes that negative stimulated calcitonin tests less than 11 years after operation do not provide assurance of cures for MTC in MEN IIa although negative tests after more than 5 years for MEN IIb are encouraging. Calculations of volume doublings accounting for various-sized tumors are compatible with Knudson's two-mutational-event theory on the initiation of neoplasia.

Adolescent↗
PubMed1983

Preparation and characterization of membrane fractions enriched in outer and inner envelope membranes from spinach chloroplasts. I. Electrophoretic and immunochemical analyses.

We have developed a fast and reliable method for the separation of two membrane fractions respectively enriched in outer and inner envelope membranes from isolated, intact, purified spinach chloroplasts kept in a hypertonic medium (0.6 M mannitol). This separation was achieved by osmotically shrinking the inner envelope membrane, thus widening the intermembrane space, and then subsequently removing the "loosened" outer envelope membrane by applying low pressure to the shrunken chloroplasts and slowly extruding them through the small aperture of a Yeda press under controlled conditions. By centrifugation of the mixture obtained through a discontinuous sucrose gradient, we were able to separate two membrane fractions having different densities (fraction 2 or light fraction, d = 1.08 g/cm3, and fraction 3 or heavy fraction, d = 1.13 g/cm3). The recent characterization of polypeptides localized on the outer envelope membrane from spinach chloroplasts, E10 and E24 (Joyard, J., Billecocq, A., Bartlett, S. G., Block, M. A., Chua, N.-H., and Douce, R. J. Biol. Chem., 258, 10000-10006) enabled us to characterize our two membrane fractions. Analyses of the polypeptides by sodium dodecyl sulfate-polyacryl-amide gel electrophoresis and immunoblotting have shown that fraction 2 (light fraction) was completely devoid of polypeptide E30, which is involved in the transport of phosphate across the inner envelope membrane, but was enriched in polypeptides E10 and E24. The reverse was true for fraction 3 (heavy fraction). Under these conditions, it is clear that fraction 2 is strongly enriched in outer envelope membrane whereas fraction 3 consisted mostly of inner envelope membrane. Indeed, by immunoelectrophoresis, we were able to demonstrate that, on a protein basis, fraction 2 contained about 90% of outer membrane, whereas fraction 3 contained about 80% of inner membrane. Further characterization of the outer envelope membrane was achieved by using thermolysin, a nonpenetrant protease.

Cell Fractionation↗
PubMed1983

Preparation and characterization of membrane fractions enriched in outer and inner envelope membranes from spinach chloroplasts. II. Biochemical characterization.

In the previous paper (Block, M. A., Dorne, A.-J., Joyard, J., and Douce, R. (1983) J. Biol. Chem. 258, 13273-13280), we have described a method for the separation of membrane fractions enriched in outer and inner envelope membranes from spinach chloroplasts. The two envelope membranes have a different weight ratio of acyl lipid to protein (2.5-3 for the outer envelope membrane and 0.8-1 for the inner envelope membrane). The two membranes also differ in their polar lipid composition. However, in order to prevent the functioning of the galactolipid:galactolipid galactosyltransferase during the course of envelope membrane separation, we have analyzed the polar lipid composition of each envelope membrane after thermolysin treatment of the intact chloroplasts. The outer envelope membrane is characterized by the presence of high amounts of phosphatidylcholine and digalactosyldiacylglycerol whereas the inner envelope membrane has a polar lipid composition almost identical with that of the thykaloids. No phosphatidylethanolamine or cardiolipin could be detected in either envelope membranes, thus demonstrating that the envelope membranes, and especially the outer membrane, do not resemble extrachloroplastic membranes. No striking differences were found in the fatty acid composition of the polar lipids from either the outer or the inner envelope membrane. The two envelope membranes also differ in their carotenoid composition. Among the different enzymatic activities associated with the chloroplast envelope, we have shown that the Mg2+-dependent ATPase, the UDP-Gal:diacylglycerol galactosyltransferase, the phosphatidic acid phosphatase, and the acyl-CoA thioesterase are associated with the inner envelope from spinach chloroplasts whereas the acyl-CoA synthetase is located on the outer envelope membrane.

Cell Fractionation↗
PubMed1983

Localization of polypeptides to the cytosolic side of the outer envelope membrane of spinach chloroplasts.

Nonpenetrating proteolytic enzymes (such as thermolysin) were used to probe the cytosolic surface of the outer envelope membrane from spinach chloroplasts. Up to 20 different envelope polypeptides were susceptible to a mild digestion of isolated intact chloroplasts by thermolysin. Most of the thermolysin-sensitive envelope polypeptides were not extracted by a mixture of chloroform/methanol (2:1, v/v). A clear exception was E10 which is hydrophobic and, in addition, is an integral membrane polypeptide. Using antibodies to envelope polypeptides sensitive (E10 and E24) and insensitive (E30 and E37) to thermolysin, we demonstrated that only antibodies to E10 and E24, but not antibodies to E30 and E37, induced agglutination of intact chloroplasts. In addition, immunofluorescence experiments demonstrated that only antibodies to E10 and E24, but not antibodies to E30 and E37, gave a green fluorescence at the outer surface of intact chloroplasts. These experiments demonstrate that E10 and E24, and probably all the thermolysin-sensitive envelope polypeptides, are accessible from the cytosolic side of the outer membrane of the chloroplast envelope.

Chloroplasts↗
PubMed1983

Thyroid nodules indeterminate by needle biopsy.

In our experience with 121 patients 18 (15 percent) thyroid nodules studied by needle biopsy were considered indeterminate relative to the presence of a low-grade, well-differentiated carcinoma. For 11 of the 18 patients, operation was performed with carcinoma identified in two (18 percent). Although experience reduced this problem, the frequency of carcinoma justifies operation for patients with indeterminate thyroid nodules by needle biopsy, unless other factors dictate otherwise. Inadequate results of fine-needle aspiration biopsy requires a determination of therapy on the basis of other clinical factors. However, permanent disappearance or great reduction in size following aspiration of cystic nodules, repeat biopsy, and biopsy with large needles are important in supporting nonoperative therapy. The indeterminate and inadequate cases must be considered in assessing reports of the use of needle biopsy of thyroid nodules. The large size of a thyroid nodule and previous external radiation therapy are factors supporting operative treatment. Improved selection of patients with benign thyroid nodules for thyroid hormone suppression therapy is needed--thyroid-releasing hormone testing may be of help.

Adenocarcinoma↗
PubMed1983

The clinical course after definitive operation for medullary thyroid carcinoma.

Thorough family studies that used the radioimmunoassay procedure for calcitonin (CT) during the past 12 years have provided data on 70 patients from 12 kindreds with hereditary medullary thyroid cancer (hereditary group) and 28 patients with sporadic or nonhereditary medullary thyroid cancer (sporadic group). Serum CT elevations after total thyroidectomy as evidence of residual tumor have been encountered in 21 of 25 patients in the hereditary group with palpable tumors preoperatively (84%) and in 13 of 24 in the sporadic group (54%). However, none of 26 patients with hereditary nonpalpable lesions found by family screening studies have been found to have postoperative CT elevations. Both cervical lymphadenectomy in patients with lymph node involvement at initial operation and reexploration in those with CT elevations after total thyroidectomy have been unsuccessful in the eradication of metastatic disease. These data emphasize the value of thorough family studies that use stimulated CT measurements in the detection of early and curable neoplasms. Involvement of the parathyroid glands or the adrenal medulla that occurred in the hereditary multiple endocrine neoplasia syndromes varied from family to family; it influenced the clinical course and required regular surveillance with calcium and catecholamine studies. Separation of hereditary from sporadic cases has practical value in the identification of family members at risk and also in the provision of prognostic data.

Adult↗
PubMed1982

Age-related probability of development of hereditary medullary thyroid carcinoma.

Hereditary medullary thyroid carcinoma is inherited as an autosomal dominant trait; at birth each child of an affected parent has a 50% chance of developing the disease. Measurement of plasma calcitonin concentrations after provocative calcium or pentagastrin stimulation has proved useful in the early diagnosis of this disease. To determine the age-related risk of conversion from a negative to a positive provocative test, 445 members of 11 kindreds were studied with sequential tests. Of 159 family members with a 50% risk at birth of developing medullary thyroid carcinoma 38 converted from a negative to a positive test result (mean age of conversion was 15 years). By means of methods previously described for determining the age-related probability for developing Huntington chorea, we present a method for determining the probability of development of medullary thyroid carcinoma. An individual at risk whose test result was negative had the following probability of converting to a positive test result at a later date: age (years)/probability, 0/0.5; 5/0.49; 10/0.41; 15/0.25; 20/0.16; 25/0.10, 30/0.05; and 35/0. We conclude that hereditary medullary thyroid carcinoma is regularly detectable in the pediatric age group and that screening should begin by age 5 years and be continued at regular intervals until age 35.

Adolescent↗
PubMed1982

Probable clonal origin of aldosteronomas versus multicellular origin of parathyroid "adenomas".

Adrenocortical adenomas causing hyperaldosteronism in two women heterozygous at the X chromosome-linked glucose-6-phosphate dehydrogenase (G-6-PD) locus exhibited only one G-6-PD isoenzyme. This finding suggests a clonal development for these benign tumors and contrasts with the multicellular origin of parathyroid adenomas reported in three patients from our institution in 1977 and found subsequently in seven other hyperparathyroid women whose cases are reported here. One of these seven patients had hereditary hyperparathyroidism. In this case each of three glands removed showed both A and B G-6-PD isoenzymes in similar ratios as were found in normal tissues. The multicellular origin of hereditary hyperparathyroidism is compatible with the concept of parathyroid lesions being manifestations of the first genetic event in Knudson's two-mutational-event theory for the initiation of cancer. The multicellular origin of sporadic parathyroid tumors suggest that they are caused by some factors stimulating many cells in the parathyroid glands. The young average age of onset of eight cases of parathyroid cancer from five families with hereditary hyperparathyroidism in the literature is also compatible with Knudson's theory. G-6-PD studies of other aldosteronomas, parathyroid tumors, and other endocrine neoplasms may provide important information about the pathogenesis of these conditions.

Adenoma↗
PubMed1981

Primary treatment of well-differentiated thyroid cancer.

Operations for well-differentiated thyroid carcinoma should be individualized on the basis of the pathologic variety and clinically demonstrated extent of carcinoma. Although precise studies have not been carried out specifically to provide treatment plans for various clinical situations, experience does provide management policies to follow currently. The basic operation is thyroid lobectomy. This appears adequate for minimal carcinoma, small (under 1.5 cm diameter) carcinoma, and low grade encapsulated follicular carcinomas of the thyroid. Extent of operation for other clinical patterns of well-differentiated thyroid carcinoma is indivualized to permit control of the disease in the neck. If well-differentiated carcinoma is recognized only postoperatively, a decision for reoperation can relate to evidence of multicentricity. Improvements in autotransplantation of parathyroid tissue and diagnostic needle biopsy have permitted improvements in therapy in recent years.

Adenocarcinoma↗