The chemical composition of microsomes and mitochondria from silver beet

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Abstract
Dispersions of whole beet petiole, and microsomes and mitochondria separated from such dispersions by differential centrifuging, were analyzed to determine the lipid, nucleic acid and protein contents. To establish the validity of the analytical procedures for determination of ribonucleic acid (RNA) and deoxyribonucleic acid (DNA), agreement between estimates based on absorption of UV light, organic phosphorus and ribose (or deoxyribose) was sought. Several published analytical procedures were shown to be unsatisfactory. Estimates of RNA based on determination of ribose were considerably above the true value. An improved analytical procedure for determination of RNA and DNA in plant materials is described. The validity of the separation of RNA and DNA was examined by estimation of uracil and thyamine isolated from hydrolysates with paper chromatography. No method of differential extraction of RNA and DNA was found to be satisfactory. The microsomes and mitochondria from beet petiole contain 2 types of RNA: 1 extractable with cold N perchloric acid, and the other not extractable with this solvent but extractable with 10% (w/v) sodium chloride at 100[degree] for 20 minutes. It is suggested that the former type is associated predominantly with Palade granules, and the latter type with structural lipoprotein material. Mitochondria and microsomes of beet petiole differ considerably in their content of lipid, particularly of phospholipid. Lipid and protein comprise over 90% of the dry weight of beet microsomes. The mitochondria and microsomes differ only slightly in their content of nucleic acids. The distribution of nitrogen, phospholipid and nucleic acids among the fractions from dispersions of beet petiole was examined. About 72% of the nitrogen and about 23% of the nucleic acid remained in the supernatant after centrifuging at 50,000 g for 90 min. They are fewer Palade granules associated with microsomes from beet petiole than with microsomes from animal tissues. This probably accounts for the lower nucleic acid content of beet microsomes as compared with microsomes from animal tissues.
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