The Condensed Matrix of Mature Chromaffin Granules

Conclusions

The important finding in the present study is not the latency per se (Fig. 1) but that the latency was highly dependent on the granule protein concentration and essentially absent at a granule concentration of 2 mg protein/mL (Fig. 2). The lack of latency at high granule protein concentration most likely reflects the decrease in homospecific DBH activity by increasing matrix protein concentration as shown in Fig. 3. As the matrix- dependent inhibition is present in an extensively dialysed matrix fraction containing molecules ≥30 kDa (defined by the preparation procedure), even after heat-treatment, we conclude that the heat-stable acidic matrix glycoproteins (chromogranins) are responsible for the inhibition. A matrix fraction largely free of chromogranin A (precipitated by 35 mM Ca²⁺ at pH 5.5) as well as a preparation of redissolved precipitated matrix proteins (enriched in chromogranin A) also inhibited sDBH. This indicates that the matrix-dependent inhibition of sDBH results from the combined effect of several proteins identified in the matrix (Fischer-Colbrie et al., 1987). As the matrix-dependent inhibition of sDBH was largely overcome by high salt concentration (Fig. 4), we suspect that electrostatic interactions between sDBH and the chromogranins (Hogue-Angeletti 1977; Helle et al., 1978) inhibit the sDBH by an unknown mechanism. It should be noted that the mechanism behind the anion-activation of purified sBDH is also unknown (Craine et al., 1973).