Stimulation and Inhibition of Milk (Lipoprotein) Lipase by Proteins from Egg Yolk Lipoproteins

In an accompanying paper we describe a method to purify from very low density lipoproteins of egg yolk, two protein fractions (T1 and T2) with cofactor activity for lipoprotein lipases. When added to media containing (bovine milk) lipoprotein lipase and Intralipid (a lecithin‐stabilized emul‐sion of soy bean triacylglycerols) they caused an immediate, several‐fold increase in the rate of lipolysis. It has been considered characteristic for lipoprotein lipases that their activity is strongly suppressed by high concentrations of NaCl. Such inhibition was seen in assay systems with whole serum, lipoproteins, or crude fractions of egg apoproteins as source of cofactor activity. In contrast NaCl caused little or no inhibition in assay systems with no cofactor source added or with T1 or T2 proteins as cofactors, indicating that salt inhibition is not exerted directly on the active site of the enzyme or on its ability to interact productively with cofactor proteins. Addition of some of the other egg apoproteins changed the response to salt of activity stimulated by T1 and T2 proteins so that it became similar to that seen in assay systems with serum or other crude sources of cofactor activity. Thus, these other proteins could inhibit the enzyme activity and this inhibition was strongly potentiated by increasing the salt concentration of the medium. Inhibition at high salt concentration was seen also with hemoglobin and with myoglobin, suggesting that it is a rather nonspecific effect. It occurred immediately on addition of the inhibitory proteins to the medium, it was reversible and was relieved in a competitive manner by increased amounts of lipid substrate or of cofactor proteins.