Lysosomes

Lysosomes are spherical membrane-enclosed organelles that are approximately 0.5 fim in diameter and contain enzymes required for intracellular digestion (Figure 1-1-6).

Lysosomes consist of two forms:

■ Primary lysosomes have not yet acquired the materials to be digested. They are formed by budding from the trans side of the Golgi apparatus.

■ Secondary lysosomes are formed by the fusion of the primary lysosome with the substrate to be degraded and have contents that are in various stages of degradation.

Lysosomes contain approximately 60 hydrolytic enzymes. These include nucleases for degrading DNA and RNA, lipases for degrading lipids, glycosides es for degrading glycoconjugates (glycoproteins, proteoglycans, and glycolipids), proteases and peptidases for degrading proteins, and a variety of phosphatases.

■ All lysosomal enzymes are acid hydrolases, with optimal activity at a pH of approximately 5.0.

• The synthesis of the lysosomal hydrolases occurs in the RER; the hydrolases are transferred to the Golgi apparatus, where they are modified and packaged into lysosomes.

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Figure [-1-6. Lysosomes

Clinical Correlate

Peroxisome Deficiency Several genetic diseases are associated with the impairment or absence of peroxisomes. These patients fail to oxidize very long chain fatty acids and accumulate bile acid precursors. The four most common disorders are:

■ Zellweger (cerebrohepatorenal) syndrome

• Neonatal adrenoleukodystrophy

• Infantile Refsum disease

• Hyperpipecolatemia

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