Recently we have shown that the substrate specificity of catalytic
IgG isolated from sera of patients with Hashimoto's
thyroiditis,
systemic lupus erythematosus (SLE),
polyarthritis and
hepatitis B for classic poly(N) homopolynucleotide substrates and for specific
tRNA(Phe) with compact and stable structure was correlated with the type of disease. At the same time the cleavage specificity was different in comparison with that of all known human RNases. Here we investigated for the first time the hydrolysis by the IgGs isolated from sera of 31 patients with different diseases of the in vitro transcript of human mitochondrial
tRNA(Lys) which has less stable structure as compared to
tRNA(Phe). The level of activity was strongly dependent on the patient, but in general increased in the order:
hepatitis B </= Hashimoto's
thyroiditis < SLE. The pH dependencies and various
salts effects also varied for Abs from the sera of different patients. Nevertheless, the
RNase activity of all IgGs was specifically stimulated by Mg(2+)
ions, that essentially completely suppress the activity of all known human RNases. In contrast to the classical substrates, no correlation between patient's
IgG cleavage specificity and a specific disease was revealed; each patient demonstrated an individual repertoire of polyclonal
RNA-hydrolyzing IgGs independently of the disease.