Hemoglobin proteolytic enzymes

Hydrolysis in dilute acid under conditions which lead to the preferential rupture of aspartyl bonds may provide an excellent means for specific cleavage of polypeptides and proteins. Of all acid degradative techniques, hydrolysis in dilute acid appears to be the most specific and closely approaches the specificity of certain proteolytic enzymes. This method, however, has not been applied widely to problems on sequence analysis. Ingram and Stretton (1962) hydrolyzed a tridecapeptide from the S-chain of human hemoglobin A2 for 12 hr at 105°C in 0.25 M acetic acid. Prefer-... 

It seems reasonable to suppose that the elevated cathepsin activity in dystrophic muscle, by enhancing muscle protein breakdown in vivo, is the cause of the increased rate of protein turnover the increased synthesis could then be seen as an adaptive response to the accelerated breakdown. There is yet, however, no real evidence that this is so the factors which control the breakdown of protein in muscle fibers are probably complex and little understood (PIO). A further possibility, that the proteins of atrophying muscles are in some way more susceptible to breakdown by proteolytic enzymes, seems unlikely Kohn (K9) reported that myosin from denervated rat muscle was digested normally by trypsin, and this was found to be true also of myosin from dystrophic mice and chickens (Kl), whereas Pollack and Bird (P21) stated that the autolytic activity of denervated muscle was not increased relative to the breakdown of hemoglobin by the muscle. 

In 1937, Balls et al. [4] purified one of the proteolytic fractions and called their crystalline enzyme papain. This was an unfortunate choice, since the name papain is also used for the crude papaya latex containing multiple enzymes. In 1939, Lineweaver and Balls published a refinement of the isolation method, and they also reported the presence in the papaya latex of another proteinase with a higher ratio of milk-clotting to hemoglobin-digesting activity than papain [5], In 1941 Jansen and Balls presented a description of a second proteolytic enzyme [6]... 

The peptide that is released following translation is not necessarily in its final functional form. In some cases the peptide is proteolytically cleaved before it becomes fimctional. S)mthesis of digestive enzymes uses this strategy. Sometimes the protein must associate with other peptides to form a fimctional protein, as in the case of hemoglobin. Cellular enz)unes add carbohydrate or lipid groups to some proteins, especially those that will end up on the cell surface. These final modifications are specific for particular proteins and, like the sequence of the protein itself, are directed by the cellular genetic information. 

Figure 1. Stability of the active acid protease at pH 8, The enzyme was incubated in 0,1 M sodium phosphate buffer, pH 8 at 37. Aliquots were removed at different time intervals and assayed for proteolytic activity at pH 2.5, using hemoglobin as substrate.

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